🚀 Example: Add Express.js tests for body and headers

[amlannandy]

Add more examples for the express-rest-api package, preferably handling more methods such as POST and covering more scenarios for testing (missing fields in req body, headers etc).

[amlannandy]

Raised this issue as I thought having just one basic test isn't enough. Unless there's some other source for getting examples now and this repo isn't being maintained anymore? Let me know if that's the case. I can raise a PR with some more examples if the maintainers think it's necessary.