Fix 204 responses, hangs and crashes with REST. Remove Content-Type and make sure no content is returned by OPTIONS request in REST interface. In write_html set the Content-Length when response is not encoded/compressed (fixes hang due to missing content-length with unencoded data). In REST interface do not raise UsageError for invalid api version. Return json error with proper message. Fixes crash.

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