1) If the response if of type None, just accept anything and do not
validate.

As per: wordnik's swagger spec, operation type for pet.addPet is void
but still the response sent back is {u'code': 200, u'message':
u'SUCCESS'}, which has no concrete structure to check against.

2) Accept Py model in request body.

Swagger-py lets you build any instance of a model without any check.
But, when it is passed as a request body param, validation checks kick
in. _flat_dict() is used to recursively convert the model into a dict
which json can dump later.

While this conversion, type is checked against the swagger spec.
If any of the attrs are None (that happens if a default instance is
created and any of the attrs are complex) they are removed from the
dict. And if they were required for that model, Error is raised.
Merge pull request #11 from Yelp/AddPyInstanceInRequestBody

Allow Python model as request body parameter.