Fixes HTTP Authentication

After the migration to AFNetworking2, HTTP Auth stopped working.
Probably self-signed certificates as well.

`connection:canAuthenticateAgainstProtectionSpace:` will only be called
if the delegate doesn't implement
`connection:willSendRequestForAuthenticationChallenge:`, which
`AFURLConnectionOperation` does.

Fixes wordpress-mobile/WordPress-iOS#2129

Possibly related issues:

- https://github.com/wordpress-mobile/WordPress-iOS/issues/1232
- https://github.com/wordpress-mobile/WordPress-iOS/issues/1933