Fix 'no response returned' RuntimeError during cancellation in BaseHT…

…TPMiddleware.

If an incoming request is disconnected before a response gets produced, don't
raise a RuntimeError.

starlette/middleware/base.py

@@ -163,6 +163,8 @@ async def coro() -> None:
             except anyio.EndOfStream:
                 if app_exc is not None:
                     raise app_exc
+                if request._is_disconnected:
+                    return Response()
                 raise RuntimeError("No response returned.")
 
             assert message["type"] == "http.response.start"