stringifyQuery fix

[Pimm]

In commit 886de38, we switched from an external alternative to Node.js' querystring module.

Unbeknownst to me, said external alternative has special logic for nested objects on which we rely in a specific endpoint. This commit adds that special logic to our codebase.

Additionally, I added a test to verify the behaviour of stringifyQuery with objects.

Fixes #165.

[dsempel]

Bug fix ✅
Unit test ✅
Nothing to add, except that I'm wondering if there might be more endpoints with nested query params. Did you check? It's theoretically possible to have even more deeply nested objects.

[Pimm]

Correct. This change only fixes this specific scenario: a first level nested object with primitive values (since it doesn't even handle arrays within such an object correctly).

From what I can tell ‒ at least by looking at our type definitions ‒ this is the only endpoint where we have something like this. So we should be OK. If it turns out we need more advanced query strings, I'll introduce smarter code.