Add response content_type(s)

[philippeauriach]

OpenAPI let responses have multiple response_types, as per https://swagger.io/docs/specification/describing-responses/

This PR adds:

• a new content_type in the response object, with a default to application/json (the default used before)
• the ability to pass several responses for the same response code, BUT with a different content_type (passing two responses with the same code + content type would raise an error, as we did before when passing two responses with the same code)

The only "issue" I see is that the response description is taken from the first response. I initially wanted to modify the response object to

class Response:
-    def __init__(self, model: type, code: int = 200, description: str = "Success", content_type: str = DEFAULT_CONTENT_TYPE):
+    def __init__(self, code: int = 200, content, description: str = "Success"):
-        self.model = model
        self.code = code
        self.description = description
+        self.content = content # content would be an array of {content_types: str, model: type}

But I feel that this would change too much the current behavior, so I preferred to "duplicate" the "description" field. I am of course open to discussion on this point !

[philippeauriach]

Hello @jakemwood, what is the process to get a pull request to review ? Sorry to tag you, but as you're the one who reviewed my previous pull request... thanks!

[jakemwood]

Sorry for the delay here, this seems to make sense! I'll get this in.