Issue title: Grape should log a warning when the formatter returns a 406 for an unsupported content type.
 Issue body: Right now when an unsupported content type is passed as the request.media_type grape returns a 406. 

I would be great if the media_type was logged along with the 406 status because when this happens in error it is really hard to debug.

 Comments: 
 Comment 0: Including the requested format and media type along in the body of the error. Let me know if this works.

 Comment 1: This returns 'txt' even when format is 'xml' for example.
Is this expected behaviour?

 Comment 2: @leods92 It wouldn't be. Error formats can be controlled separately though. Build a repro case?

 Comment 3: @dblock
By creating a MyApi class with `content_type :json, "application/json"` in it, only json is allowed. Great.
Then, you can try accessing a resource with .xml extension for example and you'll get _The requested format 'txt' is not supported._. This probably happens because Grape ignores input kind since it's not trying to parse it (because we didn't tell it to support other formats). When a user appends an extension, say `.xml` though, it's counter-intuitive to get that 'txt' message. I think that to fix this, we'd need to extract the extension from the URL. For a quick bugfix I'd just remove the format from the error message all together.
What do you think?

 Comment 4: It does sound strange, some code that determines the format should settle on XML in this case, open another issue with maybe a spec?

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