Fixed #12663 -- Formalized the Model._meta API for retrieving fields.

Thanks to Russell Keith-Magee for mentoring this Google Summer of
Code 2014 project and everyone else who helped with the patch!

django/apps/registry.py

@@ -337,7 +337,12 @@ class Apps(object):
 
         This is mostly used in tests.
         """
+        # Call expire cache on each model. This will purge
+        # the relation tree and the fields cache.
         self.get_models.cache_clear()
+        if self.ready:
+            for model in self.get_models(include_auto_created=True):
+                model._meta._expire_cache()
 
     ### DEPRECATED METHODS GO BELOW THIS LINE ###