Contourf was handling the extend kwarg by mapping the extended
ranges directly into the 0-1 normal colormap range, so that
using set_under and set_over had no effect.
The legend support for extend options was also using the
wrong names for the options.
Note that trying to simplify the code further by using np.inf a
few lines above, in self._levels, does not work; I'm not sure what
it breaks, but it definitely breaks something, probably in the