Bug report

Bug summary

As of 2.2:

plt.gca(projection="polar"); plt.gca(projection="lambert")

shows a warning and replaces the originally created polar axes by a lambert axes (not sure what behavior we actually want this to have once axes-collision detection in add_axes is removed).
However, in

plt.gca(projection="polar"); plt.gca(projection="rectilinear")

the second call to gca (using a rectilinear projection) fails to reset the projection at all and leaves the original polar axes in place.

Noted while thinking about #10674.