Enumerable#sort works even if #to_a returns a frozen Array

Ruby's documentation on Enumerable#to_a does not suggest that the returned
Array must be mutable (only that it must contain the items in the Enumerable).
In fact, Array#to_a itself can return a frozen Array. Therefore, it is better
to account for Enumerables which return a frozen Array from #to_a.

While running the Rubinius tests to make sure this change doesn't break anything, I discovered that Rubinius can't be built using itself! (At least not the 2.2.9 release.) I had to use MRI 2.1.2.