Unit tests for HTTP endpoints

We need unit tests to measure behavior, to prevent regressions and to catch issues early. Testing against live endpoints is not recommended, and testing against a localhost HTTP server is not recommended.

I think we want to use something like [`vcrpy`](https://github.com/kevin1024/vcrpy) or [`betamax`](https://github.com/sigmavirus24/betamax) to capture and replay requests from particular endpoints. We could set up mock endpoints as necessary to capture other expected situations so that they remain recorded for future test runs.

Resolving this issue doesn't require unit testing every single thing, but it does require getting a solid mocked test harness in place with unit tests for at least a few example endpoint configurations, so we can build on that test suite over time.