Added tests for min_value and max_value on a DecimalField

This adds tests for a regression where the `min_value` and `max_value`
arguments are not being set for a DRF `DecimalField` even though the
corresponding `MinValueValidator` and `MaxValueValidator` is being set
on the model fields.

Note that this only appears to be a regression for Django < 1.9, as
these regression tests pass on newer versions of Django.

tests/test_model_serializer.py

@@ -899,3 +899,29 @@ class TestDecimalFieldMappings(TestCase):
         serializer = TestSerializer()
 
         assert len(serializer.fields['decimal_field'].validators) == 2
+
+    def test_min_value_is_passed(self):
+        """
+        Test that the `MinValueValidator` is converted to the `min_value`
+        argument for the field.
+        """
+        class TestSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = DecimalFieldModel
+
+        serializer = TestSerializer()
+
+        assert serializer.fields['decimal_field'].min_value == 1
+
+    def test_max_value_is_passed(self):
+        """
+        Test that the `MaxValueValidator` is converted to the `max_value`
+        argument for the field.
+        """
+        class TestSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = DecimalFieldModel
+
+        serializer = TestSerializer()
+
+        assert serializer.fields['decimal_field'].max_value == 3