FIX: Raise errors when wrong string arguments are passed to `resample`
- [x] closes #19303
- [x] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

As a first step, the initial commit just extends the resampling tests so that they fail when a misstyped string argument, e.g. for `label`, passes silently without raising.

I have the fix for the code ready and will commit when the tests have failed once.
tests look good. pls push the fix when you can. Hello @cchwala! Thanks for updating the PR.





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

##### Comment last updated on January 21, 2018 at 15:47 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19307?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@b286789`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19307/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/19307?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #19307   +/-   ##
=========================================
  Coverage          ?    91.5%           
=========================================
  Files             ?      150           
  Lines             ?    48745           
  Branches          ?        0           
=========================================
  Hits              ?    44606           
  Misses            ?     4139           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.87% <100%> (?)` | |
| #single | `41.72% <0%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19307?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/19307/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.43% <100%> (ø)` | |

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 thanks @cchwala 