Fix issue where pk was was being set to a string.

rest_framework/mixins.py

@@ -105,6 +105,10 @@ class UpdateModelMixin(object):
         """
         # pk and/or slug attributes are implicit in the URL.
         pk = self.kwargs.get(self.pk_url_kwarg, None)
+        try:
+            pk = int(pk)
+        except ValueError:
+            pass
         if pk:
             setattr(obj, 'pk', pk)