Add IntegrationTestCase

One of our projects contains an `IntegrationTestCase`, which extends `RequestTestCase` with a call to `render()` that puts the HTML response into a `string`.  This is generally useful, and I think it should be added here.

@meshy second opinion? :)
I just don't associate `in {haystack}` with `Expected {count} instance{plural} of {needle}` when reading the sentence, even though that's intended.