Better empty-cache behavior in Event#modified_files.

It won't do for the event to raise an exception simply because nothing's been cached yet.

lib/fsevents/event.rb

@@ -17,7 +17,7 @@ def modified_files
       when :mtime
         files.select { |f|  File.mtime(f) >= stream.last_event }
       when :cache
-        cache = stream.dirs[path]
+        cache = stream.dirs[path] || {}
 
         files.select do |f|
           cached = cache[f]

spec/event_spec.rb

@@ -188,6 +188,16 @@
           modified_files.should_not include(file)
         end
       end
+
+      it 'should handle this path not yet cached' do
+        @dir_cache.delete(@path)
+        expected_files = @files
+        modified_files = @event.modified_files
+
+        expected_files.each do |file|
+          modified_files.should include(file)
+        end
+      end
     end
   end
 end