Negotiate Response Content-Type from request's Accept header.

[duckbrain]

Resolves #1148

This changes the default Context to negotiate the response Content-Type using the request's Accept header exclusively, instead of including the request's Content-Type in the negotiation. Failed negotiation will default to using render.Options#DefaultContentType. (This is important because Internet Explorer does not pass an Accept header that negotiates HTML.)

This is not backwards compatible

HTTP clients using Content-Type instead of Accept in their request headers will not work the same way. Code that overrides these values (such as those using https://github.com/gobuffalo/mw-contenttype) will no longer have the desired effect. They should be removed in favor of the DefaultContentType option or a middleware that sets the Accept header.

gobuffalo/httptest#2 is needed for tests to pass Merged!

[duckbrain]

Does this need to add something for buffalo fix to fix https://github.com/gobuffalo/mw-contenttype middleware links to a new route that Add/Set's the Accept header?