Add Proxy-Authorization header if proxy_url has username

[ngicks]

quick fix for #184

[ngicks]

Tested with this build https://github.com/ngicks/volta/tree/feat-support-proxy-authorization
under my company's proxy, it is working.

If anything else is needed I would be happy to do that.

[sbstp]

Thanks for the catch, this seems like a pretty big issue. I would probably enable this feature all the time, even if the basic-auth feature is not enabled, to avoid surprising behavior.

[ngicks]

Force-pushed fixing casing (basicAuth -> basic_auth) to keep commit history clean, I see MRs are merged without squash.
Changed base64 to be non-optional. Proxy-Authorization header is always added if proxy_url has authorization info.

[sbstp]

Yeah I use rebase merges when people take the time to make nice commits, squash otherwise.

[sbstp]

Thanks