docs: def validate(self):
          """Validate request data before sending it out. Return True/False."""
          # check if required_fields aren't present
          for field in set(self.required_fields) - set(self.request_data):
              if not isinstance(field, string_types):
                  # field was a collection, iterate over it and check by OR
                  return bool(set(field) & set(self.request_data))
              return False
          return True