BUG: List indexer on PeriodIndex doesn't coerce strings
- [x] closes #11278
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Description:
In single object indexer case,
datetime string converted to datetime-like object. Because indexing use ".xs" which is ".get_loc" that has type casting labels code. 

But in list indexer, 
there is no type check and casting code. So i add them.

this bug happen not only series but also dataframe And "datetimeindex" as well as "periodIndex"

Hello @proost! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:







There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-05-03 15:17:33 UTC @jbrockmendel if you'd have a look @jreback 
Sorry about the mess. I made mistakes while rebase on it. Again, I'm really sorry. will look at this today @proost there is some code in _libs.index that i _think_ is trying to do this for PeriodIndex but is evidently not being reached.  can you see if this can just be handled by PeriodEngine.get_indexer? > @proost there is some code in _libs.index that i _think_ is trying to do this for PeriodIndex but is evidently not being reached. can you see if this can just be handled by PeriodEngine.get_indexer?

Nope. PeriodEngine can't. @jbrockmendel over to you if any comments. looks find. can you merge master and ping o green. ping @jreback  thanks @proost for sticking with this