Support shading=None in pcolor(mesh)

[rcomer]

Rationale

If pcolor/pcolormesh receives shading=None, that should be treated the same as if shading was not set. My colleague was using pyplot.pcolormesh on a geoaxes, and pyplot explicitly defaults the shading to None. This caused a hard-to-debug artefact!

Implications

[rcomer]

Thanks @dopplershift!