Fix errors caused by `double.as_null_object.freeze` on 2.99

This fixes #698.
Now that 2.99.0 and 3.0.0 have shipped we've started putting issue/PR numbers in the changelog to provide those who care with something to go look at to find out more.  Examples here:

https://github.com/rspec/rspec-core/blob/master/Changelog.md#310-development

Can you add that here as well?