Improve Namespace.response decorator to handle HTTP/204 responses with no need to provide a Schema

[frol]

Currently, schema (model) is required for all non-error HTTP codes. There is no need to bother with models when HTTP/204 (No content) is going to be responded, so no model might be valid option in the Namespace.response decorator.

I would try to implement it as missing model, something along these lines, but you will need to change the latter code do expect None model:

- if model is None:
+ if model is None and code != 204:

As a subtask, it would be nice to find a place where HTTP/204 code is defined as constant (Flask or Werkzeug might have it).

/cc @khorolets

[khorolets]

Isn't it done in #28?