Fix use of loose falsy comparison when getting arguments in HttpContext.

This would cause problems when e.g. sending a required number arg that
equaled 0. It would not be found by HttpContext#getArgByName, so the
invocation would then fail a required check, even though 0 is
perfectly valid input.
Merge pull request #126 from STRML/fixFalsy

Fix use of loose falsy comparison when getting arguments in HttpContext.