TextHelper#simple_format attempts to call undefined raw method

`TextHelper#simple_format` attempts to use the `raw` method which is defined in the `OutputSafetyHelper` helper, but is not actually included in the module.

This is a problem when you only want to include this helper in isolation, for example:

```ruby
class MyClass
  include ActionView::Helpers::TextHelper
end

MyClass.new.simple_format('hi') #=> NoMethodError: undefined method `raw' for #<MyClass:0x007f83099a3d08>
```

I've added the tests + fix in seperate commits, feel free to cherry-pick just the implementation if the test is not required.

This issue is present in rails 4.0.2, rails 4.0.1 was fine.