Fix PHP notices that are raised in unit test suite.

[m4olivei]

Tests pass, but there are a number of PHP notices that are raised, eg.

https://travis-ci.org/chapter-three/AppleNewsAPI/jobs/192384948

These should be cleaned up.

[m4olivei]

I tried for an hour or so to come up with something that would work to test and assert PHP notices and errors, but wasn't able to. Seems like something about how the json() methods do their work, doesn't allow PHPUnit to bubble up an exception correctly to be caught by the test case. 🤷‍♂️