Update unspecified_exception_spec.rb

Some mild stubbing to cover an uncovered branch of code.

Maybe there is a better way? Or a code change?

Before submitting the PR make sure the following are checked:

[x] Feature branch is up-to-date with master (if not - rebase it).
[x] Squashed related commits together.
[x] Added tests.
[x] Updated documentation.
[x] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
[x] The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

[ ] Added the new cop to config/default.yml.
[ ] The cop is configured as Enabled: pending in config/default.yml.
[ ] The cop is configured as Enabled: true in .rubocop.yml.
[ ] The cop documents examples of good and bad code.
[ ] The tests assert both that bad code is reported and that good code is not reported.
[ ] Set VersionAdded: "<<next>>" in default/config.yml.

If you have modified an existing cop's configuration options:

rubocop / rubocop-rspec