onExit() is similar to the onLoad() callback. onExit() is called on each
module just before the process exits. This can be used to check state in
unit tests, but not to perform I/O. The process will forcibly exit as soon
as all of the onExit callbacks are made.
There is one major API change in the refactor: filename extensions are now
required when requiring or including modules.
Added extra test to test-module-loading.js.
If users do not send transfer-encoding or content-length headers, then I
will not add any additional. Content-Length: 0 is assumed if there aren't
other headers and chunked encoding is rare.
