Plugin installer proxy not working properly for HTTPS URLs

[timroes]

When specifying an http_proxy for installing a plugin and you try to install a plugin via HTTPS, you will receive an error:

Error: Client request error: socket hang up
Plugin installation was unsuccessful due to error "Client request error: socket hang up"

This is because the HTTP proxy package send to the proxy is actually build in a corrupt way.

If you e.g. are installing a plugin as follows:

https_proxy="http://192.168.178.50:808" bin/kibana-plugin install https://example.com/plugin.zip

the actual request to the proxy, will have the following HTTP content:

GET http://example.com:443/plugin.zip
Host: example.com:443

So as you can see even though it picks up the HTTPS-iness from the URL and uses port 443 (in the Host header and the URL) it will still write the http in front of the URL, thus failing, due to the mismatch of protocol and actual protocol spoken on that port.

This looks for me like an issue within the upstream library HttpProxyAgent, that generates that proxy request.

/cc @marius-dr