Motivation
When receiving certain patterns of response body parts, we can end up
recursing almost indefinitely to deliver them to the application. This
can lead to crashes, so we might politely describe it as "sub-optimal".
Modifications
Keep track of our stack depth and avoid creating too many stack frames.
Added some unit tests.
Result
We no longer explode when handling bodies with lots of tiny parts.
Co-authored-by: David Nadoba <d_nadoba@apple.com>