fix several http binding serde issues

[lucix-aws]

two issues:

• deserialization inside an httpPayload struct would still think it was in http bindings and go to read from the headers etc. rather than the body, I just replaced the flaky inBindings flag with a more stateful check
• serialization of a nested struct inside httpPayload struct would failure to properly close the object } when existing it. the only place we're supposed to skip that close is on the top-level struct when there wasn't a body, fixed with a depth check

relates aws/aws-sdk-go-v2#3504