ENH: Fix `by` in DataFrame.plot.hist and DataFrame.plot.box
- [x] closes #15079
- [x] xref:  #11053, #28177
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
