BUG: Indexing MultiIndex with Series failed.
Previously, accessing elements of a MultiIndex-indexed DataFrame with a Series
failed. This changes that behavior so that it is possible to use a Series to
access elements from a MultiIndex-indexed DataFrame, just as one would use
a list.

 - [x] closes #14730
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15041?src=pr) is 84.75% (diff: 100%)
> Merging [#15041](https://codecov.io/gh/pandas-dev/pandas/pull/15041?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **0.01%**

```diff
@@             master     #15041   diff @@
==========================================
  Files           145        145          
  Lines         51129      51141    +12   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43343      43345     +2   
- Misses         7786       7796    +10   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [74e20a0...3698087](https://codecov.io/gh/pandas-dev/pandas/compare/74e20a0c4146919a82312c1027dcb57b48b613e3...369808734faf9ce542863121cd01320225095c05?src=pr) thanks @bkandel (I moved the tests to the appropriate location)