Bug in basis caching based on multidimensional vs scalar dealias keyword

[kburns]

Looks like passing a scalar for dealias for a multidimensional basis will be correctly expanding, but only after caching, causing actually identical bases to be instantiated separately.

Ref: https://groups.google.com/g/dedalus-users/c/X0qr_S4sxzk/m/QzIsWtcMCAAJ