title: Simpleform fields_for and nested_routes not working on Rails 4
body: <p>I'm trying to get this <code>fields_for</code> on <code>simple_form</code> working with Rails 4:</p>
  
  <p>Below is my form:</p>
  
  <pre><code>= simple_form_for(hall_webcasts_path(@hall), :html =&gt; { :class =&gt; "form-horizontal" }, :wrapper =&gt; "horizontal", defaults: { :input_html =&gt; { class: "form-control"}, label_html: { class: "col-lg-4" } } ) do |f|
    = f.error_notification
    = f.simple_fields_for @webcast do |b|
      .form-inputs
        = b.input :name
        = b.association :hall, prompt: "Choose a hall", :input_html =&gt; { class: "chosen" }
        = b.input :webcast_type
        = b.input :media_type
        = b.association :user, prompt: "Choose a user", :input_html =&gt; { class: "chosen" } if current_user.has_role? :admin
        = b.input :user, as: :hidden, :value =&gt; current_user.id if !current_user.has_role? :admin
        = b.input :details
        = b.association :template, :input_html =&gt; { class: "chosen" }
        = b.input :background_colour, :input_html =&gt; { "colourpicker" =&gt; "true" }
        = b.input :start, as: :string, :input_html =&gt; { class: "datetimepicker form-control" }
        = b.input :finish, as: :string, :input_html =&gt; { class: "datetimepicker form-control" }
  
      .form-actions
        = b.button :submit, class: "btn btn-primary"
  </code></pre>
  
  <p>Below is my routes:</p>
  
  <pre><code>    resources :venues, shallow: true do
          #Halls
          get "hall/:id/exhibition" =&gt; "halls#exhibition", as: :exhibition
          get "hall/:id/visit" =&gt; "halls#visit", as: :hall_visit
          get "structure", :to =&gt; "venues#venue_structure"
          resources :asset_types, :booths_tags, :tags, :uploaded_files, :events, :chats
          resources :halls do
              resources :webcasts
              resources :booths do
                  resources :chats
              end
          end
      end
  </code></pre>
  
  <p>This however always create:</p>
  
  <pre><code>/halls/1/webcasts/new
  </code></pre>
  
  <p>In my form action and something like below:</p>
  
  <pre><code>/halls/1/webcasts[webcast][name]
  </code></pre>
  
  <p>in my input fields <code>name</code> attributes.</p>
  
  <p>Is there a way to fix this issue?</p>
  
  <pre><code>class Webcast &lt; ActiveRecord::Base
    belongs_to :hall
    belongs_to :user
    belongs_to :template
  
    validates :name, :hall, :user, :template, presence: :true
  
  end
  </code></pre>
  
  <p>I also added <code>webcast</code> model for reference</p>