Ninja send unexpected content type

[jeanNimb]

It seems, that a character encoding is added to the response content type, even there is no character encoding set.
For example, if no character encoding is set, ninja delivers as response the content type application/gzip;charset=UTF-8. But expected is application/gzip.
Since this could cause some problems (e.g. with 3rd party libraries), this should be fixed.

My proposed solution by not falling back to UTF-8, if no character encoding is set or if it was cleared explicitly.