BUG: Fixed font size (set it on both x and y axis). #8765
Fix for https://github.com/pydata/pandas/issues/8765 
The font size was only set on x axis if vertical or the y axis if horizontal.

@TomAugspurger can you review this?
 @papaloizouc ready to merge if you could add a short note to `doc/source/whatsnew/v0.15.2`.
 Thanks!
