Get tests running on django 1.11

I'm a little concerned that this loses some of the efficiencies (in
particular, chunking) from previous versions. That's something that can
probably be improved.

polymorphic/query.py

@@ -25,6 +25,25 @@ except ImportError:
 Polymorphic_QuerySet_objects_per_request = CHUNK_SIZE
 
 
+if django.VERSION >= (1, 9):
+    # We ignore this on django < 1.9, as ModelIterable didn't yet exist.
+    from django.db.models.query import ModelIterable
+
+
+    class PolymorphicModelIterable(ModelIterable):
+        def __iter__(self):
+            base_iter = super(PolymorphicModelIterable, self).__iter__()
+
+            if self.queryset.polymorphic_disabled:
+                for o in base_iter:
+                    yield o
+                return
+
+            real_instances = self.queryset._get_real_instances(base_iter)
+            for obj in real_instances:
+                yield obj
+
+
 def transmogrify(cls, obj):
     """
     Upcast a class to a different type without asking questions.
@@ -72,6 +91,9 @@ class PolymorphicQuerySet(QuerySet):
         # to that queryset as well).
         self.polymorphic_deferred_loading = (set([]), True)
         super(PolymorphicQuerySet, self).__init__(*args, **kwargs)
+        if django.VERSION >= (1, 9):
+            # On django < 1.9 we override the iterator() method instead
+            self._iterable_class = PolymorphicModelIterable
 
     def _clone(self, *args, **kwargs):
         # Django's _clone only copies its own variables, so we need to copy ours here
@@ -407,6 +429,8 @@ class PolymorphicQuerySet(QuerySet):
 
         return resultlist
 
+    if django.VERSION < (1, 9):
+        # On django 1.9+, we can define self._iterator_class instead of iterator()
         def iterator(self):
             """
             This function is used by Django for all object retrieval.