Add HttpClient abstract class

[ob-stripe]

r? @brandur-stripe @richardm-stripe

(Targets ob-request-object until #878 is merged. You probably want to review #878 first.)

Renames HttpClient to HttpURLConnectionClient (because this client uses java.net.HttpURLConnection to make HTTP requests), and adds a new HttpClient abstract class.

This will make it easy to provide alternative implementations, e.g. to use a different HTTP library or to write a mock client.

I went with an abstract class rather than an interface because I expect that HttpClient will have some concrete methods. For instance, the request retry logic should be applicable regardless of the concrete HTTP client implementation.

The diff is hard to read because git/GitHub failed to pick up that HttpClient was renamed to HttpURLConnectionClient :/

[richardm-stripe]

Is the hope here to enable users to substitute their favorite HTTP library? Or is it strictly for developers of stripe-java.

[richardm-stripe]

Regardless,
lgtm

[ob-stripe]

Thanks Richard!

Is the hope here to enable users to substitute their favorite HTTP library? Or is it strictly for developers of stripe-java.

Both. There hasn't been a ton of demand for being able to use alternative HTTP libraries, but some users have been asking for an easier way to mock requests in their Stripe integration tests.

[brandur-stripe]

Great to finally see some movement on this Olivier! Nice one.