API: Fix using the same module as generator and prop

Change Ie2dee41e introduced a bug when using the same module as both
generator and prop, e.g.
?action=query&titles=Main_Page&generator=categories&prop=categories

The generator should be an uncached instance of the module.

Change-Id: I80c8b3a715f266a61614c1e1d02aa80b72643ec8
Merge "API: Fix using the same module as generator and prop"