Fix url doesn't re-produce #27

fix issue https://github.com/strongloop/loopback-testing/issues/27
If the url param is a function, it will be use to produce an url in
every test case.

Signed-off-by: Clark Wang <clark.wangs@gmail.com>
Is this depending on a `beforeEach` hook that sends the request before each test?

In that case the naming is very misleading, a better name would be `should retrieve the expected model in the first test`, `should retrieve the expected model in subsequent tests`.