Prevent "double-prefixing" Mr Bean generated classes

[cowtowncoder]

In case of generic type instantiation, prefix currently (as of 2.11.1) gets applied twice in some cases.
This is due to there being 2 parts: first an intermediate abstract base class is created to bind type variables, and then the actual concrete class is generated. But package prefix is effectively applied twice; this should not occur.

But to reduce risk of regression let's change this for 2.12 and not in 2.11 patch.