[WIP] Fix for PHP 5.5 unit tests (and XDebug >= 2.2.0)

This PR aims to fix issues observed with unit tests on PHP 5.5. One differentiating factor observed is a behavior change in XDebug 2.2.0 -- `xdebug_get_headers` no longer resets itself between calls. As such, response sender tests need to alter expectations based on the XDebug version detected.

Travis is also still showing errors in the Zend\Filter\Compress\Zip tests, which unfortunately I cannot reproduce locally. I'm considering disabling these tests when in Travis.