HTTP2 streams terminated prematurely on GOAWAY

I've been debugging a networking problem  in https://github.com/owncloud/client/issues/7020#issuecomment-465578496 and I think the issue is in Qt's HTTP2 stack.

The symptom is QNetworkReply emitting finished() with NoError even though the request hasn't properly finished. A warning like 'stream 15 finished with error: ""' is seen.

Analysis (from mitmproxy talking to the server, so not entirely reliable, but shows what's going on):

• Client opens HTTP2 connection
• Requests data on streams 1, 3, 5, 7, 9
• Stream 1, 9 finish; Stream 3, 5, 7 are still streaming DATA frames and are not finished
• There's a GOAWAY 2^31-1 no-error frame from the server
• mitmproxy closes the connection with a tls close_notify
• there are more DATA frames from the server on streams 3, 7 that aren't ACKed by the client
• finally a GOAWAY 9 no-error frame, and the server's close_notify

If I read qhttp2protocolhandler.cpp correctly the problem is that the GOAWAY with a lastStreamID of 2^31-1 causes Qt to terminate all running streams. Specifically:

    } else if (lastStreamID >= nextID) {
        // "A server that is attempting to gracefully shut down a connection SHOULD
        // send an initial GOAWAY frame with the last stream identifier set to 2^31-1
        // and a NO_ERROR code."
        if (lastStreamID != Http2::lastValidStreamID || errorCode != HTTP2_NO_ERROR)
            return connectionError(PROTOCOL_ERROR, "GOAWAY invalid stream/error code");
        lastStreamID = 1; // <- causes all streams to be closed
    } else {

The GOAWAY spec says that the streams should not be closed at this point.