BUG: rolling_count() and expanding_*() with zero-length args; rolling/expanding_apply with min_periods=0
Closes https://github.com/pydata/pandas/issues/8056
Closes https://github.com/pydata/pandas/issues/8080

rebase this
 Rebased.
 this is fine,

can you fix the docs in whatsnew here:

http://pandas-docs.github.io/pandas-docs-travis/whatsnew.html
(the change about the `center`)

you need to adjust the `` somewhere
 Fixed, I think. I guess it doesn't like non-whitespace following closing double quotes.
 ok, ping on green
 Green.
 Updated to fix #8080.
 thanks, merged
