Speed up String#truncate

Benchmark:
          user     system      total        real
old   1.550000   0.040000   1.590000 (  1.585866)
new   1.250000   0.040000   1.290000 (  1.287693)

Author: amatsuda

activesupport/lib/active_support/core_ext/string/filters.rb

@@ -50,6 +50,6 @@ def truncate(truncate_at, options = {})
         length_with_room_for_omission
       end
 
-    "#{self[0...stop]}#{options[:omission]}"
+    "#{self[0, stop]}#{options[:omission]}"
   end
 end