Refine `assertJSONEquivalent` so that it is more forgiving.

[mihaelamj]

The function assertJSONEquivalent that is only used in the testing suite, should be a bit more forgiving when comparing JSON strings. Here's a ghist demonstrating a failing test for JSON strings that should be considered equivalent.

https://gist.github.com/mihaelamj/de29510a1444c3ab307a2cc7fd284c9f

Here's another ghist demonstration of how to adapt the codable model from a previous one, so that the comparison would evaluate to equivalent:

https://gist.github.com/mattpolzin/b8cf851cb2d61321909ce7d54d3f0a1e