Fix another regression in rake task pattern handling.

Apparently `FileList[file_list_object]` works, and some
users used `task.pattern = FileList[…]`.

See https://github.com/rspec/rspec-core/commit/667a4ca00f50fddb3fca7cdfcaf97633ba267b1f#commitcomment-7725799
Merge pull request #1701 from rspec/fix-another-rake-task-pattern-regression

Fix another regression in rake task pattern handling.