some test fixes

changed package.json to use `jest-cli` and not `jest`
added jest-related configs in `package.json` that will circumvent the "function" error you were seeing, but will yield a different error.
added a test that allows you to completely mock the `body-parser` module.
Thanks @awei01 that is all insightful and helpful.  I'm happy that I now know how to test more complicated modules using Jest and I'm hoping that Jest'll be more robust with these kinds of modules in the near future. :)  I think I'll close the issue given that a) there are workarounds as you've given, and b) there's another issue that should help towards this also.  Do you agree?  I should add also my very many thanks for your attention and effort helping with this issue, it's very inspiring to see this happening with strangers across the Internet. :)