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

[tarebyte]

This PR addresses two issues.

• Webmock was updated and validates the response body. A Hash is not listed valid parameter (see Error on invalid body 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 #95

[afeld]

Thanks!