Fix another regression in rake task pattern handling.

This was reported by @aprescott: https://github.com/rspec/rspec-core/pull/1671#issuecomment-54623310.

The regression was originally introduced in #1653.
See also #1691 and #1692.
Merge pull request #1694 from rspec/fix-rake-task-pattern-regression-2

Fix another regression in rake task pattern handling.