BUG: Dense ranking with percent now uses 100% basis
- `DataFrame.rank()` and `Series.rank()` when `method='dense'` and
  `pct=True` now scales to 100%.

See #15630

 - [ ] closes #15630 
 - [ ] tests added / passed
 - [X] passes ``git diff upstream/master | flake8 --diff``
 - [X] whatsnew entry
