Allow JSON coersion for error responses

I removed the test for unexceptional-status from :auto response body coesion because the content-type says already application/json and JSON error responses are not impossible.

I have a server which responses with a well defined JSON structure in the case of an error. The content type of such responses is also correctly set to application/json.

I found the related issue [:as :json may hide status code errors with parse errors](https://github.com/dakrone/clj-http/issues/71) where you introduced the unexceptional-status test. This may be necessary for :as :json but I don't think its helpful for :as :auto.