Include Beefcake injections for nested types.

Explicitly ensure that the preamble for generated classes and their
nested types include Beefcake's injected generated class behavior.

Commit ec102f8cf37626827a395a856758b970c47898ca introduced a
regression whereby said nested classes lost these injections.

The canonical example from https://developers.google.com/protocol-buffers/docs/overview
no longer worked.