Support for raw buffer payloads + file attachments (instrumenting superagent)
Rework some unit tests to be integration tests, using the actual shims

It's hard to test superagent otherwise, given we don't have access to the
superagent request object inside our tests.
Mixed supertest / superagent tests

Since the divide is quite arbitrary:
we get the request/response info where it's the easiest
Simplify instrumenting so module can focus on replacing their functions
Fix return values of shimmed functions