Make test fail when a request changes

This is a follow up to #80 /cc @tmelliottjr

In #80 we changed the way field values are unset, by using a new mutation instead of setting the value to `""`. We expected the existing test to unset a text field would fail as the implementation changed, but it didn't. We agreed that it would be good if it would fail, so that updating the recorded fixtures would be necessary explicitly in a situation like this.