fix #1429 by correctly handling tags with dots

This was a very old regression. This used to work before the rails 3.2 upgrade... Added tests 
to prevent future unnoticed breakage

test/unit/tagging_test.rb

@@ -11,6 +11,6 @@ class TaggingTest < ActiveSupport::TestCase
     
     tagging.destroy
     
-    assert_nil Tag.find_by_name("hello")
+    assert_nil Tag.find_by_name("hello"), "Tag should be destroyed when last use in tagging was removed"
   end
 end