BUG: rolling.quantile does not return an interpolated result
Now computing the quantile of a rolling window matches the default of the rolling method in Series, DataFrame and np.percentile.

Fixes bugs #9413 and #16211

 - [x] closes #9413
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry
