Close HTTP loop when connection task cancelled (#2245)

* Terminate loop when no transport exists

* Add log when closing HTTP loop because of shutdown

* Add unit test

sanic/server/protocols/http_protocol.py

@@ -109,7 +109,7 @@ class HttpProtocol(SanicProtocol, metaclass=TouchUpMeta):
         except Exception:
             error_logger.exception("protocol.connection_task uncaught")
         finally:
-            if self.app.debug and self._http:
+            if self.app.debug and self._http and self.transport:
                 ip = self.transport.get_extra_info("peername")
                 error_logger.error(
                     "Connection lost before response written"