+ http: support proxy authentication
= http: refactor auth headers
- move Challenge to AdditionalRules
- move Www-/Proxy-Authenticate to SimpleHeaders
- move Proxy-Authorization to AuthorizationHeader
- delete ProxyAuthenticateHeader, ProxyAuthorizationHeader, WwwAuthenticateHeader
Merge pull request #451 from 2beaucoup/feature/proxy-auth

+ http: support proxy authentication