RKObjectMapping copyWithZone should use initWithClass

Fixes issues with copying RKObjectMapping where classes and subclasses are setting defaults in the designated initializer.
Fixes crash with RKTableController which blocks use of init.

This could be a fix that should be done on RKTableController to work with the fact that RKObjectMapping copyWithZone uses init. If so, please comment & close this pull request and I'll fix it over there.