SetContentType middleware breaks Content-Type detection for requests

The documentation for `SetContentType` only mentions responses and says nothing of requests:

> SetContentType is a middleware that forces response Content-Type.

However, this breaks Content-Type detection for requests in the `DefaultDecoder` because `SetContentType` and `GetRequestContentType` share the same context key.

It's not obvious to me why `GetRequestContentType` even looks at the context, as that seems to betray the function name. But if that behavior is going to stay, I suggest using a separate context key so that `SetContentType` does not interfere.