QtWebsockets client fails handshake if there is delay between response status line and headers

If there is a delay between status line ("HTTP/1.1 101 Upgrading Protocol") and headers, QWebSocketPrivate::processHandshake erroneously detects server disconnect. The problem is that disconnect condition is checked using QAbstractSocket::atEnd function which just checks if data available and not the actual socket state. This results in multiple error signals:

websocket error: QWebSocket::processHandshake: Connection closed while reading header.
websocket error: Invalid statusline in response: Date: Thu, 11 Feb 2016 20:07:37 GMT^M .
websocket error: Invalid statusline in response: Server: WSGIServer/0.1 Python/2.7.10^M .
websocket error: Invalid statusline in response: Upgrade: websocket^M .
websocket error: Invalid statusline in response: Connection: Upgrade^M .
websocket error: Invalid statusline in response: Sec-WebSocket-Accept: oQ+NK56UdGrsXFisB9chjE3SU+Y=^M .
websocket error: Invalid statusline in response: Sec-WebSocket-Version: 13^M .
websocket error: Invalid statusline in response: Content-Length: 0^M .
websocket error: Invalid statusline in response: ^M .

You can reproduce bug using attached server.py. It simulates HTTP server and introduces delay between status line and headers.

Attached patch fixes the problem.