Httpbun Mixer – build simple test/mock APIs for testing HTTP clients

Httpbun is an HTTP server similar to httpbin.org, but has a few more bells. One of them is the `/mix` endpoint, and the Mixer tool.

With httpbin, we can't have an endpoint that responds with, say, 400 status code, _and_ an HTML body with a script injection, for when I need to test any HTTP client. This is possible with `/mix`. Here's the Mixer URL for such an endpoint: https://httpbun.com/mixer/s=400/b64=PHNjcmlwdD5hbGVydCgiaW5q....