code: def _to_json(self, strip, to_serialize=None):
          
          if to_serialize is None:
              to_serialize = copy.copy(self.__dict__)
          pkcs12_val = to_serialize.get(_PKCS12_KEY)
          if pkcs12_val is not None:
              to_serialize[_PKCS12_KEY] = base64.b64encode(pkcs12_val)
          return super(ServiceAccountCredentials, self)._to_json(
              strip, to_serialize=to_serialize)
docs: Utility function that creates JSON repr. of a credentials object.
  
          Over-ride is needed since PKCS#12 keys will not in general be JSON
          serializable.
  
          Args:
              strip: array, An array of names of members to exclude from the
                     JSON.
              to_serialize: dict, (Optional) The properties for this object
                            that will be serialized. This allows callers to
                            modify before serializing.
  
          Returns:
              string, a JSON representation of this instance, suitable to pass to
              from_json().