Handle HTTPS_PROXY in http.Transport

[David-Crty]

what

• Adding the possibility to properly handle the HTTPS_PROXY env var

why

• In some infra context the remote GitHub endpoint (eg: api.github.com is accessible only through a remote proxy)
• Currently if you set HTTPS_PROXY, the variable is ignored

references

• Other PR that also tries to resolve this issue but has a conflict Reuse default transport that handles proxy configuration #16

Let me know if can do more to get this feature merged

[David-Crty]

I have implemented the change, @goruha.