BUG:reorder type check/conversion so wide_to_long handles str arg for…
closes #22468
 
- [x] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

#22468 

This reorders the list type check/conversion to occur before the stubname membership test. This will prevent the ValueError in the cases where a column name is a substring of a stubname when a string is passed to the stubnames argument of wide_to_long().

Also, this is my first PR and I love pandas! @csmcallister 

* Great to hear you love `pandas`!
* Good start, but we're going to need a test and `whatsnew` entry as well Of course! I can get to those things later this day. Thank you so much for pointing me in the right direction. Hello @csmcallister! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/reshape/melt.py`](https://github.com/csmcallister/pandas/blob/32c3a644fcde278b1970874076967b508384b0a7/pandas/core/reshape/melt.py) !

 - There are no PEP8 issues in the file [`pandas/tests/reshape/test_melt.py`](https://github.com/csmcallister/pandas/blob/32c3a644fcde278b1970874076967b508384b0a7/pandas/tests/reshape/test_melt.py) !



##### Comment last updated on September 07, 2018 at 00:48 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=h1) Report
> Merging [#22490](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0f656f7934ae3935002b0dfe2bc5a88d93ae9ed6?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22490/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22490   +/-   ##
=======================================
  Coverage   92.04%   92.04%           
=======================================
  Files         169      169           
  Lines       50773    50773           
=======================================
  Hits        46734    46734           
  Misses       4039     4039
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.45% <100%> (ø)` | :arrow_up: |
| #single | `42.23% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/melt.py](https://codecov.io/gh/pandas-dev/pandas/pull/22490/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZWx0LnB5) | `97.34% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=footer). Last update [0f656f7...58df814](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=h1) Report
> Merging [#22490](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1a798be98b7a605693c6d1143ab1d703a77f47a1?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `0%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22490/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22490      +/-   ##
==========================================
+ Coverage   92.04%   92.04%   +<.01%     
==========================================
  Files         169      169              
  Lines       50783    50782       -1     
==========================================
+ Hits        46741    46744       +3     
+ Misses       4042     4038       -4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `100% <ø> (+9.54%)` | :arrow_up: |
| #single | `42.26% <0%> (+0.04%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/melt.py](https://codecov.io/gh/pandas-dev/pandas/pull/22490/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZWx0LnB5) | `97.34% <0%> (ø)` | :arrow_up: |
| [pandas/tslib.py](https://codecov.io/gh/pandas-dev/pandas/pull/22490/diff?src=pr&el=tree#diff-cGFuZGFzL3RzbGliLnB5) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/io/formats/latex.py](https://codecov.io/gh/pandas-dev/pandas/pull/22490/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvbGF0ZXgucHk=) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/lib.py](https://codecov.io/gh/pandas-dev/pandas/pull/22490/diff?src=pr&el=tree#diff-cGFuZGFzL2xpYi5weQ==) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/22490/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/parser.py](https://codecov.io/gh/pandas-dev/pandas/pull/22490/diff?src=pr&el=tree#diff-cGFuZGFzL3BhcnNlci5weQ==) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/io/sas/sas\_constants.py](https://codecov.io/gh/pandas-dev/pandas/pull/22490/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfY29uc3RhbnRzLnB5) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/types/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/22490/diff?src=pr&el=tree#diff-cGFuZGFzL3R5cGVzL2NvbmNhdC5weQ==) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/tseries/plotting.py](https://codecov.io/gh/pandas-dev/pandas/pull/22490/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvcGxvdHRpbmcucHk=) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/tseries/converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/22490/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvY29udmVydGVyLnB5) | `100% <0%> (ø)` | :arrow_up: |
| ... and [129 more](https://codecov.io/gh/pandas-dev/pandas/pull/22490/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=footer). Last update [1a798be...32c3a64](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Well, this is embarrassing. Copying and pasting jacked up the indentation and angered pep8speaks. Weird since because beforehand I ran ```git diff upstream/master -u -- "*.py" | flake8 --diff``` and nothing printed out. looks good. can you rebase on master and push again. ping on green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=h1) Report
> Merging [#22490](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4cc0a71c729aaf9a5cf46a2c4b3fd5e0a4fd37a9?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22490/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22490   +/-   ##
=======================================
  Coverage   92.18%   92.18%           
=======================================
  Files         169      169           
  Lines       50820    50820           
=======================================
  Hits        46850    46850           
  Misses       3970     3970
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <100%> (ø)` | :arrow_up: |
| #single | `42.38% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/melt.py](https://codecov.io/gh/pandas-dev/pandas/pull/22490/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZWx0LnB5) | `97.34% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=footer). Last update [4cc0a71...1fb07b4](https://codecov.io/gh/pandas-dev/pandas/pull/22490?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Rebased on master, squashing commits into one. @jreback  not sue why failing, can you merge master and try again Just merged and trying again. thanks @csmcallister 