Fix str / repr for StripeObjects with decimals & add support for plain dicts

[xavdid-stripe]

Why?

We added support for native decimal types in #1769 but our code for stringifying objects for printing didn't account for it. (API requests handled it as expected). To fix, we cenralized our serialization logic so that both paths (which both end up sending things to JSON) use the same logic.

While I was in there, the user requested being able to get a JSON-dumpable dict that isn't yet a string. Since we have all the code to do that already, I added an arg to support it.

While I was in here, I simplified the way we're encoding things into JSON. Typechecks in python are a little expensive, so I tried to minimize the number of places we do those checks. I think there's more to do here, but it's internal, so there's no rush.

What?

• Reuse existing JSON encoding logic in to_dict
• add for_json kwarg to to_dict()
• tweaked encoding logic to share a little more code

See Also

#1779

[mbroshi-stripe]

Looks good!