Don't filter out the DecimalValidator if it is not supported

Previously, all validators set on a DecimalField in Django would be
stripped when converted to a Django REST framework field. This was
because any validator that was an instance of `DecimalValidator` would
be removed, and when `DecimalValidator` wasn't supported (so it was
`None`), all validators would be removed.

This fixes the issue by only removing the `DecimalValidator` instances
if the `DecimalValidator` is supported.

rest_framework/utils/field_mapping.py

@@ -130,7 +130,7 @@ def get_field_kwargs(field_name, model_field):
 
     # Our decimal validation is handled in the field code, not validator code.
     # (In Django 1.9+ this differs from previous style)
-    if isinstance(model_field, models.DecimalField):
+    if isinstance(model_field, models.DecimalField) and DecimalValidator:
         validator_kwarg = [
             validator for validator in validator_kwarg
             if DecimalValidator and not isinstance(validator, DecimalValidator)