Send the data as string.

        payload: Payload must be utf-8 string or unicode,
                  if the opcode is OPCODE_TEXT.
                  Otherwise, it must be string(byte array)

        opcode: operation code to send. Please see OPCODE_XXX.

def send(self, payload, opcode=ABNF.OPCODE_TEXT):
        """
        Send the data as string.

        payload: Payload must be utf-8 string or unicode,
                  if the opcode is OPCODE_TEXT.
                  Otherwise, it must be string(byte array)

        opcode: operation code to send. Please see OPCODE_XXX.
        """

        frame = ABNF.create_frame(payload, opcode)
        return self.send_frame(frame)