Renderer negotiation: media_type specificty evaluation weak

The `DefaultContentNegotiation` handler uses

For example: Google Chrome sends an Accept-header of `Accept:text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8`, when I request a *.png URL.
After matching the media-types with the available renderers (in my case only a custom `PNGRenderer` with a `media_type='image/png'`), only `*/*;q=0.8` is left, which happens to have the same length as the "image/png" media-type defined by the renderer (9 characters).

The specificity of the renderer's media-type over the Accept-header's one is only determined by length.
Using your `_MediaType.precedence` would be preferable in my eyes.

Regards, Fabian

rest_framework/negotiation.py

@@ -2,6 +2,7 @@ from django.http import Http404
 from rest_framework import exceptions
 from rest_framework.settings import api_settings
 from rest_framework.utils.mediatypes import order_by_precedence, media_type_matches
+from rest_framework.utils.mediatypes import _MediaType
 
 
 class BaseContentNegotiation(object):
@@ -48,7 +49,8 @@ class DefaultContentNegotiation(BaseContentNegotiation):
                 for media_type in media_type_set:
                     if media_type_matches(renderer.media_type, media_type):
                         # Return the most specific media type as accepted.
-                        if len(renderer.media_type) > len(media_type):
+                        if (_MediaType(renderer.media_type).precedence >
+                            _MediaType(media_type).precedence):
                             # Eg client requests '*/*'
                             # Accepted media type is 'application/json'
                             return renderer, renderer.media_type