BUG: Fix filter method so that accepts byte and unicode column names
- [x] closes #13101
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @Licht-T! Thanks for updating the PR.







Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on November 21, 2017 at 16:25 Hours UTC Thanks @jreback, now fixed! # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18238?src=pr&el=h1) Report
> Merging [#18238](https://codecov.io/gh/pandas-dev/pandas/pull/18238?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c4a2cd362afdfbc21cb62a8e09758226e86735c4?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `73.33%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18238/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18238?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18238      +/-   ##
==========================================
- Coverage   91.36%   91.34%   -0.03%     
==========================================
  Files         164      164              
  Lines       49733    49743      +10     
==========================================
- Hits        45439    45436       -3     
- Misses       4294     4307      +13
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.14% <73.33%> (-0.01%)` | :arrow_down: |
| #single | `39.61% <13.33%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18238?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/18238?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.73% <100%> (ø)` | :arrow_up: |
| [pandas/compat/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/18238?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9fX2luaXRfXy5weQ==) | `58.18% <60%> (+0.08%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18238?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18238?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18238?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/18238?src=pr&el=footer). Last update [c4a2cd3...3c58fcb](https://codecov.io/gh/pandas-dev/pandas/pull/18238?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @Licht-T very nice!