test: adds a test for undefined value in setHeader
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 <f9f03903429036ef609ed7ca822fddfa76155c29@vagg.org>
Reviewed-By: Brendan Ashworth <2a35967cb8cf8bc6bb9ebf86d18bb6cabb303f5b@me.com>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node