Fix type error + Use Sorted Keys Where Possible

[designatednerd]

Based on #624 and some of the failures we saw when @dmandarino tried to add a test, this fixes #623.

In this PR:

• Switch [String: AnyHashable] to [String: Any] to allow for proper encoding of types like booleans
• Add a convenience method to use .sortedKeys when generating JSON on platforms where that's possible
• Adapt @dmandarino's tests from [Fix] GET Request are failing due to variablesKey cast that always fails. #624 to use a stable-sorted json structure where possible, and add test that can run on both older and newer operating systems.

/me shakes fist at sky for .sortedKeys only being on iOS 11 and above

[designatednerd]

@dmandarino wanna give me a ✅if you feel this is good or feedback if you have it?

[dmandarino]

@designatednerd Looks good to me. I've tested here too using the project I'm working on and everything looks fine and passed :D