test: fix test-http-content-length

Previously the test did not allow the last request to complete.

Fixes: https://github.com/iojs/io.js/pull/1137
PR-URL: https://github.com/iojs/io.js/pull/1145


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Bert Belder <bertbelder@gmail.com>