HTTP: Integration Testing

I've been brought up to believe that whenever you fix a bug, you should add a test for it, so it doesn't re-surface. I didn't include a test with issue #1845, because it was hard to test at a component level, and that's been bothering me.

This patch includes a test for #1845 (in HttpIntegrationSpec). It uses WireMock to simulate a system-under-test, and initialises just enough of Gatling to test it. There's also a re-usable support class, to enable this integrated style of testing.

I can't decide if this is a good approach or not. More testing's good, but it's adding complexity.

Thoughts?