network auto tests: Squid does not return a reply for requests >= 64K

"tst_qnetworkreply ioPostToHttpFromSocket:128k+1+proxyauth" times out, because the Squid proxy does not send a reply if there is proxy authentication involved and the request content length is >= 65536 (=64*1024).
The requests are sent correctly according to Wireshark, i.e.
1. client sends HTTP request
2. server sends 407
3. client sends HTTP request with proxy auth credentials
4. nothing happens

If the content length is 1 byte less than above, the server sends a 200 OK as 4.
This seems to be a problem with Squid, and has never worked, neither in Qt4 nor Qt5.