IndexAsBlog now renders a proc passed to title/body methods in view context

Resolves issue #1890 so that the documentation works for the following scenario:

```ruby
ActiveAdmin.register Post do
  index as: :blog do |post|
    title do |post|
      span(class: :my_title) { post.title + " From Block" }
    end
    body :body # calls post.body
  end
end
```