2.3.x doc scalatest

The instructions as to how to use ScalaTest are now outdated.
I've updated that and also added an example usage of testing a POST request with a JSON body.

Hope this helps!

PS - I've branched off the `2.3.x` branch, but it looks here as if github is creating the pull request off your `master` branch; not sure how to fix this (or even whether it matters).