Return void from beforeunload handler to allow window unload

Signed-off-by: Nathan Sobo <nathan@github.com>

src/window-event-handler.coffee

@@ -164,7 +164,8 @@ class WindowEventHandler
     else
       @applicationDelegate.didCancelWindowUnload()
 
-    confirmed
+    # Returning any non-void value stops the window from unloading
+    return true unless confirmed
 
   handleWindowUnload: =>
     @atomEnvironment.destroy()