Fix pattern for absolute paths

Support absolute path patterns.

While this wasn't officially supported previously, setting `rake_task.pattern` to an absolute path pattern in RSpec 3.0 and before worked since it delegated to `FileList` internally (but now just forwards the pattern on to the `rspec` command).

Fixes #1721.
Would this be sufficient?

```ruby
pattern =~ /\A[A-Z]:\\/ || pattern.start_with?(File::AltSeparator)
```

Would there ever be a windows path that starts with a single File::AltSeparator that is not an absolute path?