fix: Replace _get_val_from_obj with value_from_object (#1980)

Co-authored-by: Peter Dermott <[email protected]>

Author: petedermott

mezzanine/core/fields.py

@@ -94,7 +94,7 @@ def validate(self, value, instance):
             raise ValidationError(error)
 
     def value_to_string(self, obj):
-        value = self._get_val_from_obj(obj)
+        value = self.value_to_string(obj)
         return ",".join(value)