def sendHeader(self, key, val):
        """
        Keep sent headers so we can inspect them later.
        """
        self.factory.sent_headers[key.lower()] = val
        xmlrpc.QueryProtocol.sendHeader(self, key, val)

        Keep sent headers so we can inspect them later.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_xmlrpc.py