PlotGeometry fails to plot multiple fill colors

[dcamron]

PlotGeometry fails to plot multiple fill values as of v1.6. See v1.6 example vs v1.5 example. Reproduced locally, and I see we don't have any image tests with lists of fill colors. I've yet to dive in, but I don't imagine anything too terrifying hiding in there.

[kgoebber]

It is also failing in the stroke color as well.

[kgoebber]

Okay, so I investigated this a bit and the issue comes in updating the kwargs. In this instance we are iterating through values and so the first time through it is setting the default value of the dictionary if it did not exist in the mpl_args attribute, but then can't update the value past that point. In no other of the declarative pieces do we loop through values for plotting purposes, so we might need a unique solution for this portion of the code to ensure that a value given in mpl_args overrides something put in for a given attribute.