Use https_proxy environment variable for secure connections

[j-a-n]

Use https_proxy environment variable for secure connections and do not fallback to http_proxy.
Fixes #927

[engn33r]

I agree with the update. Although the user is expected to set is_secure to true when https_proxy is set, the updated code makes it more clear and foolproof that is_secure controls whether http_proxy or https_proxy is chosen. This avoids the case of falling back to http_proxy if https_proxy is not set when the user specifically sets is_secure to true.

Thanks for adding test coverage 👍

[j-a-n]

Thanks for merging 👍