Add spec ensuring Enumerable#sort works even if #to_a returns a frozen Array

The commit comment says it all:

Ruby's documentation on Enumerable#to_a doesn't suggest that it must return a
mutable array; it just says the Array will contain the items in the Enumerable.
And MRI's Array#to_a *can* return a frozen Array. MRI's Enumerable#sort works
even if #to_a returns a frozen Array. This spec will help other Ruby
implementations to maintain compatibility in this area.
I realize this is a bit late, but considering the issue talks about an error being raised, could the spec description be changed to `doesn't raise an error if ...`? The term "fail" could imply that perhaps no error is raised but instead the data is not sorted, or some other part doesn't quite behave the way it should. Basically this is to be very explicit about what is expected of the test.