Fixed broken test for Django < 1.9

This test was incorrectly checking that there were no validators set in
older versions of Django, even though it should have been checking for
the two validators that were set up on the model field level.

The originally regression test that this fixes was added in
7d79cf3
when fixing an issue with the `DecimalValidator`.

Author: kevin-brown

tests/test_model_serializer.py

@@ -884,7 +884,7 @@ class Meta:
 
         serializer = TestSerializer()
 
-        assert len(serializer.fields['decimal_field'].validators) == 0
+        assert len(serializer.fields['decimal_field'].validators) == 2
 
     @pytest.mark.skipif(DecimalValidator is None,
                         reason='DecimalValidator is available in Django 1.9+')