Update guzzle test

Just verified with a debugger that this codepath is actually taken and the variables are set properly, aka. its always NULL

When trying to upgrade to 8.1.7 through Composer, running "composer update drupal/core" results in guzzlehttp/guzzle updating to 6.2.0, not 6.2.1. You need to also run "composer update guzzlehttp/*" to get guzzlehttp/guzzle to 6.2.1. It's unclear to me why this is the case. Should Drupal's composer.json require "guzzlehttp/guzzle": "^6.2.1" to make sure Composer selects the right version?

Saw this problem reported by larowlan at https://twitter.com/larowlan/status/755187917711040512 and verified this was a problem with all D8 sites I am working on that had been updated through Composer.

Sharing discussion from this thread: https://twitter.com/larowlan/status/755187917711040512

Seems like the issue is that if you are 8.1.5, then composer.lock will have guzzlehttp/psr7 set to 1.3.0. guzzlehttp/guzzle 6.2.1 requires guzzlehttp/psr7 to be set to 1.3.1 When running composer update drupal/core, Composer can satisfy the version requirement for guzzle of ~6.2 with 6.2.0 and avoid breaking the lock on psr7, while 6.2.1 requires breaking the lock on both guzzle and psr7. That seems to be the explanation that makes the most sense.

Since this fixed, I've created https://www.drupal.org/node/2768953 to discuss.