Fixed #20094 - Be more careful when checking for Iterator

Python 2.6 has some different behaviour when checking
isinstance(foo, collections.Iterator).

tests/utils_tests/itercompat.py

@@ -0,0 +1,11 @@
+from django.test import TestCase
+
+from .models import Category, Thing
+
+
+class TestIsIterator(TestCase):
+    def test_regression(self):
+        """This failed on Django 1.5/Py2.6 because category has a next method."""
+        category = Category.objects.create(name='category')
+        Thing.objects.create(category=category)
+        Thing.objects.filter(category=category)