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? :)
This could easily be misused - if you want to pass `view_args` but don't specify `expected_status` it will be accidentally overriden. I think making `expected_status` a keyword only argument is appropriate.