Enable logging of expected failure cases (JSON Parseable 400s)

[twohlix]

It would be useful to document expected failure cases, 400 errors, and their response bodies.

Maybe with a required (in rspec for example) document: true declaration on the test.

[twohlix]

Just figured out that this isn't happening in my project because the error body isn't constructed in the controller action under test, its constructed by a responder and the body is blank for avocado during its json? check so it will never document.