This makes `ActionDispatch::Controller#content_type` return not only
the MIME type, but also in some circumstances the charset.
Example:
```ruby
reponse.content_type == 'text/html; charset=utf-8'
```
The MIME type-only fragment can now be accessed using `#media_type`.
Changes to the tests are not stricly necessary (because no charset is
present in the actual value), but represent the intent better.
