Remove quality parameter from chosen media type in WebFlux responses

[bclozel]

When processing requests and working on the Content Negotiation for WebFlux Controller handlers, HandlerResultHandlerSupport:

• get the list of accepted media types from the request
• get the list of producible media types from the handler
• select the most specific, concrete media type and use it for the response

It seems that when doing so, WebFlux does not remove the quality parameter from the media type, which means that the response can have a response header like Content-Type: application/json; q=0.9.

This quality parameter is properly removed in AbstractMessageConverterMethodProcessor for Spring MVC, we should do the same here.