Unit tests for express

- Splits the ``header`` & ``body`` check into different files within the ``libs/`` folder
- Adds unit tests around the ``header`` check ensuring that:
  - ``statusCode``
  - ``Content-Type``
- Adds unit tests around the ``body`` check ensuring that it can deal with:
  - ``Arrays``
  - ``Objects``
  - ``Strings``

Starts to address https://github.com/apibyexample/abe-express/issues/5