Document and ensure legacy HostingServiceClient.json_*() result types.
Review Request #2267 — Created Sept. 24, 2021 and updated
This updates the documentation for the `HostingServiceClient.json_*`() methods (`json_post()`, `json_get()`, `json_delete()`) to specifically say that header keys/values are native strings. It also adds unit tests that guarantee this as well. While here, some issues with existing unit tests were also fixed. This will help with ensuring proper usage without any confusion over types. Testing Done: Unit tests passed on Python 2 and 3. Reviewed at https://reviews.reviewboard.org/r/11645/