test: adds a test for undefined value in setHeader

As a result of 979d0ca8 there is a new check for undefined values on
OutgoingMessage.setHeader. This commit introduces a test for this case.

PR-URL: https://github.com/iojs/io.js/pull/970


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>