'Create a Message instance assembled from json fragments.
Args:
header_json (JSON) :
metadata_json (JSON) :
content_json (JSON) :
Returns:
message'

def assemble(self, header_json, metadata_json, content_json):

    header = json_decode(header_json)
    if ('msgtype' not in header):
        log.error('Bad    header    with    no    msgtype    was:    %r', header)
        raise ProtocolError("No    'msgtype'    in    header")
    return self._messages[header['msgtype']].assemble(header_json, metadata_json, content_json)
