Rewrite loadjson to use the modern function API

[ekohl]

Summary

This also resolves a bug where JSON.parse returned a StringIO. To properly catch this, the tests are rewritten to avoid mocking where possible. Exceptions are with external URLs and where a failure is expected.

Related Issues (if any)

Fixes #1414

Checklist

• [x] 🟢 Spec tests.
• [ ] 🟢 Acceptance tests.
• [ ] Manually verified. (For example puppet apply)