make media_manager a property of MediaEntry in mixin.py

In all cases where get_media_manager(_media_type_as_string) was called in
our code base we ultimately passed in a "MediaEntry().media_type" to get
the matching MEDIA_MANAGER. It so makes sense to make this a function of
the MediaEntry rather than a global function in mediagoblin.media_types and
passing around media_entry.media_type as arguments all the time.

It saves a few import statements and arguments. I also made it so the
Media_manager property is cached for subsequent calls, although I am not too
sure that this is needed (there are other cases for which this would make
more sense)

Also add a get_media_manager test to the media submission tests. It submits
an image and checks that both media.media_type and media.media_manager
return the right thing. Not sure if these tests could not be merged with an
existing submission test, but it can't hurt to have things explicit.

TODO: Right now we iterate through all existing media_managers to find the
right one based on the string of its module name. This should be made a simple
dict lookup to avoid all the extra work.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

mediagoblin/tests/test_submission.py

@@ -28,7 +28,7 @@ from mediagoblin.tests.tools import get_test_app, \
     fixture_add_user
 from mediagoblin import mg_globals
 from mediagoblin.tools import template
-
+from mediagoblin.media_types.image import MEDIA_MANAGER as img_MEDIA_MANAGER
 
 def resource(filename):
     return resource_filename('mediagoblin.tests', 'test_submission/' + filename)
@@ -197,6 +197,19 @@ class TestSubmission:
         assert 'Sorry, I don\'t support that file type :(' == \
                 str(form.file.errors[0])
 
+
+    def test_get_media_manager(self):
+        """Test if the get_media_manger function returns sensible things
+        """
+        response, request = self.do_post({'title': u'Balanced Goblin'},
+                                         *REQUEST_CONTEXT, do_follow=True,
+                                         **self.upload_data(GOOD_JPG))
+        media = self.check_media(request, {'title': u'Balanced Goblin'}, 1)
+
+        assert_equal(media.media_type, u'mediagoblin.media_types.image')
+        assert_equal(media.media_manager, img_MEDIA_MANAGER)
+
+
     def test_sniffing(self):
         '''
         Test sniffing mechanism to assert that regular uploads work as intended