[Proxy] Lower case support for proxy environment `http_proxy` and `https_proxy `

[JounQin]

Is your feature request related to a problem? Please describe.

According to the comment at #170 (comment), I open this issue to track.

Describe the solution you'd like

https://unix.stackexchange.com/questions/212894/whats-the-right-format-for-the-http-proxy-environment-variable-caps-or-no-ca

The proxy environment lower_case and UPPER_CASE should be supported at the same time.

Besides, should ALL_PROXY and all_proxy be supported too?

Describe alternatives you've considered

N/A

Additional context

N/A

Can you submit a pull request?

Yes, I'd like to help if you agree to support them.

[erezrokah]

Related to #1265

[jfsiii]

https://github.com/Rob--W/proxy-from-env is great for this! You call one function and it Does The Right Thing ™️ and returns the string to use