Bug report

Bug summary

Per title and example below.

Code for reproduction

boxplot(np.random.rand(100), medianprops={"c": "red"})

Actual outcome

/usr/lib/python3.7/site-packages/matplotlib/cbook/__init__.py:1725: UserWarning: Saw kwargs ['c', 'color'] which are all aliases for 'color'.  Kept value from 'color'
  seen=seen, canon=canonical, used=seen[-1]))

and the median line doesn't use "red", but the default provided by the rcparams.

Expected outcome

no warning and a red median line.

Matplotlib version

• Operating system: linux
• Matplotlib version: 3.0.2/master
• Matplotlib backend (print(matplotlib.get_backend())): any
• Python version: 3.7
• Jupyter version (if applicable): no
• Other libraries: no