Add an acceptance test for the logic provided by service:head-data

[bartocc]

PR ember-learn/guides-source#1980 on ember-learn/guides-source removes a unit test on service:head-data

This service is not provided by ember-learn/guides-source, but rather by empress/guidemaker and as such, should be tested here.

It should be tested with an acceptance test though, not a unit test, and the logic in test https://github.com/empress/empress-blog/blob/master/tests/acceptance/meta-test.js could should probably be replicated.

This issue is here to remind us that this test should be added at some point, since this service won't be tested anymore once ember-learn/guides-source#1980 gets accepted.