Fix NTLM Proxy authentication issues.

Issue 1) Http request's user credential is used for proxy credential.
For most use cases, user will have proxy credential stored in the keychain.
Look for credential in keychain first, then use the http request credential.

Issue 2) From wireshark network traces, calling startRequest twice does not
actually send the NTLM proxy auth challenge response.  The third startRequest
will send the auth challenge response and get past the proxy authentication.