Per-route content negotiation, and OpenAPI only for root (#693)

* WIP: remove non-openapi root spec

* Refactor ContentType

Different endpoints will favor one type over another

* Permit different Accept headers per endpoint

* Lint

* Add charset to Content-Type only when used as a header

Keep it out of error messages

* Accept: */* is last resort, not first

* Changelog

* makeMimeList consistently

* Remove schema description from OPTIONS response

test/Feature/CorsSpec.hs

@@ -69,4 +69,4 @@ spec =
         liftIO $ do
           simpleHeaders r `shouldSatisfy` matchHeader
             "Access-Control-Allow-Origin" "\\*"
-          simpleBody r `shouldSatisfy` not . BL.null
+          simpleBody r `shouldSatisfy` BL.null