Automatic request retries

[ob-stripe]

r? @brandur-stripe @remi-stripe
cc @stripe/api-libraries

Add support for automatic request retries.

The implementation is based on stripe-go, so the retry conditions are the same: requests are retried on connection errors (timeouts) and 409's. Given the recent discussions in e.g. stripe/stripe-node#559, let me know if you want me to change the conditions (e.g. to retry on 500's).

[brandur-stripe]

One comment below, but looks great!

ptal @ob-stripe

[brandur-stripe]

It might be worth adding 503s here and non-POST 500s as recently came into stripe-node. I think that's the direction we're going, and we just haven't changed stripe-go yet.

[ob-stripe]

I've updated ShouldRetry to also retry on 503's and non-POST 5xx's, similar to stripe-node.

I also realized there were a few important bits missing, which I also updated:

• we now set an idempotency key for all POST requests, if one is not already provided by the user (it would have been pretty dangerous to ship automatic retries without this!)
• non-timeout connection errors are reported as HttpRequestExceptions. These are now handled like timeout errors.
• I've also improved error handling to be a bit more resilient with unexpected bodies (I was getting NullPointerExceptions in some of my tests because I was returning {} as the body and the error handling code tries to access the error key in the JSON hash)

ptal @brandur-stripe

[brandur-stripe]

• we now set an idempotency key for all POST requests, if one is not already provided by the user (it would have been pretty dangerous to ship automatic retries without this!)

Phew! Glad you caught that one before shipping, haha.

LGTM!