Remove Connection: close header for 501 error handling rest Ralf doesn't know why the "Connection: close" header was added. Was there since the original code to support OPTIONS verb was added. He agrees it can be removed. Enable test for that code path.

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