proxy authentication for master

[key2peace]

trafficstars

tested with 3cx

[ollien]

FWIW, I have a private fork of this library which uses this patch, and I had great success with it on a server that required Proxy-Authentication. Would be cool to see it in the stable branch :)

[avocadio]

@tayler6000 I'm mindful of all the work that is being done in the v2 and likely release, but would you mind reviewing this PR - it works for me as well.

[the-master-student]

Hi everyone,

I am new to the topic and feel a bit helpless. I have checkout these changes, however I still get a proxy authorization error. How would you register and initiate a call with those changes you did on this branch?

Thank you in advance and grateful for any help.