proxy-authenticate support

[w2l0o5]

Hackney just support basic proxy-authenticate, but this is not enough. We know that many corporation proxy server do NTLM or Kerberos or Negotiate proxy authenticate, rather than basic authenticate. So maybe you can consider add a option {proxy-auth-type}, selectivily make a request header.

Header = [{<<"Proxy-Authorization">>, <<ProxyAuthType/binary, Credentials/binary>>}]

Or can implement automatically proxy authenticate, but that would be quite complicated. ;) This is just my personal thoughts!

[benoitc]

mmm good idea!

I think it implies to find the right way to handle the different way to negociate with the proxy, so a proxy can be just a backend. WIll have a look