Allows tokens to be refreshed via a proxy

[cmoman]

Hi.
I was using your library to access a webbased API via a proxy server. The proxy requires authentication. I found I couldn't refresh the token without adding the proxies parameter to the QAuth2Session class.
I have no idea if this raises other security issues but it works for my useage. I thought I'd see if was useful in the general sense by creating this pull request.
Thanks,
Chris

[Lukasa]

I think this change is entirely reasonable. =) Of course, you could have just set the proxies on the session itself (sess.proxies = proxy_dict), but there's nothing wrong with doing this as well.

[cmoman]

Thanks for the comment about setting the session proxy. I hadn't thought to do this.
However, I reverted my changes and tried your suggestion(sess.proxies = proxy_dict) but it didn't seem to work. Although the value self.proxies was set in the session class, this didn't seem to translate across into the .refresh_token() method.