NTLM proxy authentication is broken

In Qt 5, NTLM (and NTLM SSPI) proxy authentication seems to be broken.

Tested using a custom Qt 5 build from git (qtbase rev 6c879b9b7c5099455a29c95b24d6cb7e69baff99).

Description and observations:

• I am connecting using a HTTP proxy that requires NTLM authentication
  (browsers work ok, both with NTLM SSPI and manually-provided username+password)
• HTTP(s) connections from Qt do not work, not with QNetworkManager
  nor directly with QTcpSocket
• Qt doesn't even try to authenticate using NTLM if I don't provide
  some credentials for QAuthenticator in proxyAuthenticationRequired slot. (just closes the connection when 407 is received, whereas documentation says that it should automatically try the system login+password)
• When correct username and password are manually given to
  QAuthenticator (in proxyAuthenticationRequired slot), the proxy gives
  200 OK but there is no traffic after that.
• Trying to set QAuthenticator user and password to empty strings does
  not trigger SSPI, it just makes Qt send NULLs as username and
  password

After applying the original NTLM SSPI patches from Aron Rosenberg on top of Qt 4.8.3, the authentication works, although I still have to provide empty username and non-empty password in order to get SSPI working.