set method doesn't include user set timestamp if update set() is supposed to update the record with the key if it already exists. It does update the value marshalled data blob. However it doesn't update the timestamp column for rdbms tables if provided. This change updates the x_time column with the provided __timestamp or preserves the original timestamp.

try:
    # Python 3+.
    from xmlrpc import client, server
    server.SimpleXMLRPCDispatcher
except (ImportError, AttributeError):
    # Python 2.
    import xmlrpclib as client  # noqa: F401
    import SimpleXMLRPCServer as server