File.exists?("broken_symlink") returns true

``` ruby
require 'fileutils'
File.open("nowhere.rb", "w") { |f| f.write "" }
FileUtils.ln_s("nowhere.rb", "link_to_nowhere.rb")
FileUtils.rm("nowhere.rb")
puts File.exists?("link_to_nowhere.rb")
```

returns `false` on MRI `1.9.3` - `2.1`, returns `true` in JRuby `1.7.13` - `1.7.15`
