helpers: fix usage of mocha contexts

Mocha 1.19 introduced nested contexts, where each `describe` has its
own context inheriting from parent's context via prototype.

The helper `whenCalledRemotely` was reading `this.url` to allow
users to override the url. However, since `whenCalledRemotely` was
not clearing `this.url` properly at the end of the test, the override
worked for the first request only. Subsequent requests used the same
URL as provided in the helper parameters.

This commit fixes the problem by changing `this.url = undefined`
to `delete this.url`, so that the the sub-sequent calls can access
`this.__proto__.url` in the same way as the first call.
Merge pull request #16 from strongloop/feature/upgrade-mocha-to-1.9

helpers: fix usage of mocha contexts
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.