Do not overwrite https.Agent certificates

[chrmarti]

[isc-bsaviano]

@chrmarti Does this change mean that extensions who want to make HTTPS requests need to get the list of certificates themselves? Or will the proxy still provide the certificates if the https.Agent from the extension doesn't provide any?

[chrmarti]

@isc-bsaviano The latter: This issue fixes the case where the request options have an https agent with certificates. The bug is that in this case system certificates were still added to the request. The fix is to not add system certificates, so only the certificates on the agent are used. The fixed behavior will be similar to setting the certificates on the request options.