Add keep-alive support

[gajus]

[gajus]

Refer to this issue for supporting information.

https://www.reddit.com/r/node/comments/bhj53t/global_httphttps_proxy_for_nodejs_v12/

[jolivaSan]

Hi @gajus,

So I understand that 'global-agent' will use keepAlive: 'false' even if we are using a custom instance of an Agent, right? So in the case that you mention in the documentation

This behaviour allows to intercept requests of libraries that use a custom instance of an agent per default (e.g. Stripe SDK uses an http(s).globalAgent instance pre-configured with keepAlive: true).

Even if the Stripe SDK is setting up the keepAlive: true global-agent is changing to keepAlive: 'false' right?

Best Regards.