Behave more like rails 3 fixes #15

[grosser]

We are upgrading our app, and having to make the code work in rails_xss and rails 3 is pretty hard, so this makes them exactly the same,

currently with rails_xss
foo.gsub! -> boom

Now and in Rails 3
foo.gsub! -> only unsafe

@josevalim

[grosser]

(also includes pull #16 since I actually tested the code ;) )