helpers: Improve ACL asserts

Modify `_it.shouldBeAllowed` to include the actual status code
in the assertion error.

Modify `_it.shouldBeDenied` to assert the exact value of the status
code. Expect the status code configured via `app.get('aclErrorStatus')`,
add a parameter allowing callers to override this value.
helpers: add parameter for request body

Allow tests to send arbitrary request body in the requests being tested.
Merge pull request #15 from strongloop/feature/improve-access-checks

helpers: Improve ACL asserts