gRPC: fixed handling of padding on DATA frames. The response size check introduced in 39501ce97e29 did not take into account possible padding on DATA frames, resulting in incorrect "upstream sent response body larger than indicated content length" errors if upstream server used padding in responses with known length. Fix is to check the actual size of response buffers produced by the code, similarly to how it is done in other protocols, instead of checking the size of DATA frames. Reported at: http://mailman.nginx.org/pipermail/nginx-devel/2021-March/013907.html