Enable persistent connections by default

[ob-stripe]

r? @rattrayalex-stripe

This PR enables persistent connection by default. From my laptop in Europe, running test/flows.spec.js takes 33 seconds with this PR, vs. 1 minute with the master branch.

Tagged as WIP, let me know what you think!

[rattrayalex-stripe]

Loving this, so simple.

Let's test:

• The http-vs-https logic, both in makeRequest and setHttpAgent.
• That requests reuse the same connection.
• That you can still override agent (I assume there exists a test for this already).

[rattrayalex-stripe]

Another thought: we should probably add explicit docs to the README mentioning that this is the default, and telling you how to turn it off.

Maybe something like this:

---

Persistent Connections

As of stripe-node vX.Y.Z, a persistent https connection is kept open by default. This reduces latency when multiple requests are made from the same instance of stripe-node.

If needed, you can disable this behavior by providing your own https agent:

const https = require('https');
stripe.setHttpAgent(new https.Agent());

---

On the other hand, it might be better not to document this in the README off the bat, so that if users run into problems with it, they open an issue so we can find out about it and fix the problem for everyone.

[ob-stripe]

@rattrayalex-stripe I added a test for setHttpAgent, but I'm unsure how to go about testing makeRequest and that connections are actually reused. Any suggestions?

[rattrayalex-stripe]

Eh, yeah, probably not worth it. Whatever we would do there would be pretty heavily dependent on our http mocking library (eg; nock). There isn't much complexity here so I don't see it as a likely candidate for accidental regression.

I think this LGTM!

[ob-stripe]

Thanks Alex!

[ob-stripe]

Released as v6.32.1.