Fix NTLM Proxy authentication issues.

I found two issues when authenticating against NTLM Proxy on Mac OSX.  I am testing AsiHttpRequest with Microsoft Forefront TMG server as the proxy.

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.

There must be a different root cause for not responding for the NTLM auth challenge with the original code.  By making the 3rd startRequest call, I can workaround this issue, however.