Missing support for *_proxy environment variables

[fbourigault]

Guzzle is missing lowercased version of proxy variables. According to https://unix.stackexchange.com/a/212972, the *_proxy variables used to be named in lowercase.

Is this intentional from guzzle?

If no, I will submit a PR to add support for lowercased *_proxy variables.

[sagikazarmark]

Good question, I have no idea if it's intentional. I will have to check, but currently I am more focused on releasing 6.3.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed after 2 weeks if no further activity occurs. Thank you for your contributions.