Generics, as documented in pydantic's docs do not work.

Repro: Use the example from pydantic's docs. The typevar type does not get validated.

For example: Response[int](data="not an int") does not raise an error.

Furthermore, json schema generation is broken, as described here: #1002

This PR:

• fixes generics, so they actually validate (instead of silently passing)
• includes thorough tests (based on an example from pydantic's docs)