Add a test for HTTPS proxy

[jirimoravcik]

We recently added a new option ignoreProxyCertificate. We should add tests covering it.

How to do it (roughly):

1. Run a plain Node.js HTTP server that would just return Hello world.
2. Use e.g. https://www.npmjs.com/package/local-ssl-proxy to wrap the server with a self-signed SSL certificate.
3. Run a proxy-chain server that would point to the HTTPS endpoint created in 2.
4. Send a normal request via the proxy-chain proxy.
5. If ignoreProxyCertificate is set to true, you should get Hello world, otherwise you'll get a SSL error about being unable to verify the leaf signature.