500 error when not specifying JSON content type header

[aquiporras]

See #79 for more context.

When the content type header is not present in the request specifying it is JSON, the body is not properly parsed and it will fail with different errors depending on the endpoint called.

Usually with a 500 error thrown by lightning internally, which is really misleading.

Being a rest client would be nice to do more validation on the parameters (returning 400 or 422 when mandatory parameters are missing) and 415 when the content type is not JSON as expected.

[saubyk]

@aquiporras this will be a significant change, as each controller would need to updated to return contextual error.

I will keep this issue open, but will be able to address it over a period of time gradually. Meanwhile, PRs are welcome. 😄