Tox passenv by default only passes HTTP_PROXY but not http_proxy

[poggenhans]

According to the docs, tox passenv is case insensitive. Apparently this is not the case for variables passed through by default. This means that e.g. HTTP_PROXY will be forwarded by default, but not http_proxy. This is a problem because some linux programs look for the upper-case version (e.g. pip) and some for the lower-case version (e.g. apt and git).

This problem can be worked around by explicitly setting passenv = HTTP_PROXY HTTPS_PROXY, because then tox will also include the lower-case pendants, but this feels a bit counter-intuitive.

I'd propose to either include the lower-cased proxy variables into the list of default variables (and clarify in the docs that case-insensitivity doesn't apply for default variables) or treat all default variables as case insensitive. I can also prepare a PR for any of the two solutions if desired.

[jugmac00]

As @poggenhans volunteered to create a PR, @gaborbernat, which of the two suggested solutions do you prefer?

[gaborbernat]

I think this only matters on UNIX so on those envs should pass all env vars as lower and upper case. On windows env vars case insensitive. So let's do that.