Use to_json instead of hand written JSON strings for specs, and fix Webmock error.

This PR addresses two issues.
- Webmock was updated and validates the response body. A Hash is not listed valid parameter (see bblimke/webmock#427)
- Handwriting json objects as strings is pretty cumbersome and not very readable. Let `to_json` do the work for us, so that we can just make a hash

This was partially extracted from education/teachers_pet#95