FileTask: Return a late timestamp instead of an early timestamp if file doesn't exist

This should fix issue #286 which I reported earlier this week.

I modified the test method `TestRakeFileTask#test_existing_file_depends_on_non_existing_file` to confirm that the old behavior was incorrect (since it only built one of the two files and not the other) and also to catch any future regressions that cause the old behavior to come back.

By the way, `Rake::LATE > Rake::LATE` and `!(Rake::LATE <= Rake::LATE)`, but `Rake::EARLY` had similar strange properties and I don't see a huge need to fix it.

I see that `Rake::EARLY` is still being used in other parts of Rake, so I did not remove it.