Don't directly export (augmented) supertest

This is in preparation of replacing the supertest() function with
something else.
Initialize fn_args to undefined

Keeps IDEA from complaining.
Whitespace fixes in tests
Set Error object's stack trace to a request dump

This keeps track of whatever the "current" request is, and whenever
assert() returns an error, sets that error's .stack property to a nice
dump of the request and response that are "current" at that time.

This feels pretty hackish, but works surprisingly well.