Encode the common request envelope

def _encode_message_header(cls, client_id, correlation_id, request_key,
                               version=0):
        """
        Encode the common request envelope
        """
        return struct.pack('>hhih%ds' % len(client_id),
                           request_key,          # ApiKey
                           version,              # ApiVersion
                           correlation_id,       # CorrelationId
                           len(client_id),       # ClientId size
                           client_id)