Use RST_STREAM(NO_ERROR) in case server early respond (#633)

[erebe]

Http2 Server are allowed to early respond without fully consuming client input stream, but must respond with an error code of NO_ERROR when sending RST_STREAM. Nginx treat any other error code as fatal if not done so

Commit change error code from CANCEL to NO_ERROR, when the server is early responding to the client

https://github.com/hyperium/h2/issues/633 https://trac.nginx.org/nginx/ticket/2376