let acceptable types specify their quality

[wojcikstefan]

This way, we allow applications server-side to define what is the most desirable mime type in cases where */* was requested.

As RFC 7231 Section 5.3.1 says:

This weight is referred to as a "quality value" (or "qvalue") because the same parameter name is often used within server configurations to assign a weight to the relative quality of the various representations that can be selected for a resource.