Re-write request Content-Type when decoding

[florimondmanca]

Refs #23

This pull request makes it so that applications see Content-Type: application/json (instead of Content-Type: application/x-msgpack) in requests.

The motivation is to make content and Content-Type consistent from the point of view of the application, which may do additional consistency checks, eg for security purposes. FastAPI 0.65.2+ has such a check that prevents a CSRF vulnerability when a client sends JSON data with text/plain, which is exempted from CSRF checks.

Still pondering, but I think this might be an acceptable, perhaps necessary option. The idea behind msgpack-asgi is to serve as a "msgpack-to/from-JSON gateway" afterall.

cc @einfachTobi — I'd be happy to hear what you think about this. :-)

[einfachTobi]

I think it's the best approach. You never can tell, what any application will do with the request. I'm sure changing the content-type after decoding gives best compatibility with any application. At least this is also the consequent way because technically the content-type is application/json after passing the middleware.

[florimondmanca]

OK, let's roll.