Add a test case for previous_changes

[rspeicher]

This is attempting to reproduce an odd behavior we're seeing in GitLab at https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5107#note_122281994.

The problem we're seeing may be an odd interaction between ActiveRecord and some other gem we're using, but I want to first verify it's not an issue with acts-as-taggable-on itself.

This may be related to https://github.com/mbleigh/acts-as-taggable-on/issues/475#issuecomment-441038310.