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.