'Receive string data(byte array) from the server.
return value: string(byte array) value.'

def recv(self):

    (opcode, data) = self.recv_data()
    if (six.PY3 and (opcode == ABNF.OPCODE_TEXT)):
        return data.decode('utf-8')
    elif ((opcode == ABNF.OPCODE_TEXT) or (opcode == ABNF.OPCODE_BINARY)):
        return data
    else:
        return ''
