Fix potential duplicate generation of payload types.

[raphael]

This commit does two things:

1. It makes sure that action definitions that refer to user types to
   define the payload only duplicates the definition if needed (i.e. if
   the payload definition overrides some of the type attributes).
2. It makes sure that a payload type is only generated if there is not
   a user type with the same type name.

[raphael]

Fix #677