fix(HTTPRangeNotSatisfiable): Don't allow media type to be set

This patch removes the unnecessary media_type param, since 416 doesn't
normally include a body in the response. This change made some code
in the API helper, compose_error_response, unnecessary, so that function
was reworked as well.

Closes #277
Merge pull request #282 from kgriffs/issues/277

fix(HTTPRangeNotSatisfiable): Don't allow media type to be set