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"
