don't force close() when headers is already sent

Author: nggit

tremolo/http_server.py

@@ -130,7 +130,7 @@ async def _handle_response(self, func, kwargs):
             data = await agen
 
             if data is None:
-                response.close()
+                response.close(keepalive=response.headers_sent())
                 return
 
             if not isinstance(data, (bytes, bytearray, str, tuple)):