Fix modal_dialog

An empty dialog is displayed, as shown in the following example.

````
<%= content_tag :a, "Modal", :href => "#modal", :class => 'btn', :data => {:toggle => 'modal'} %>

<%= modal_dialog :id => "modal",
         :header => { :show_close => true, :dismiss => 'modal', :title => 'Modal header' },
         :body   => { :content => 'This is the body' },
         :footer => { :content => content_tag(:button, 'Save', :class => 'btn') } %>
````