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.
I did some research I think this: `pattern =~ /\A[A-Z]:\\/ || pattern.start_with?(File::AltSeparator+File::AltSeparator)` as windows network paths (unc paths) can look like this: `\\server\path`