title: undefined method `values_at' using nested_form gem
body: <p>I am getting the error</p>
  
  <pre><code>ActionView::Template::Error (undefined method `values_at' for nil:NilClass):
  </code></pre>
  
  <p>when using nested_form gem </p>
  
  <p>Here is my code</p>
  
  <p>Models</p>
  
  <pre><code>class Poll &lt; ActiveRecord::Base
  
    attr_accessible :question, :poll_answers_attributes
    has_many  :poll_answers
    accepts_nested_attributes_for :poll_answers
  end
  
  class PollAnswer &lt; ActiveRecord::Base
    belongs_to  :poll
    attr_accessible :answer
  end
  </code></pre>
  
  <p>View </p>
  
  <pre><code> =nested_form_for [:admin, @poll], mutipart: true, class: "form-horizontal" do |f|
      .span6
        .control-group
          =f.label :question, class: "control-label"
          .controls
            =f.text_field :question, rows: "5", class: "span5"
        = f.link_to_add "Add a Answer", :poll_answers
        =f.submit "Create Post", class: "btn btn-primary"
  </code></pre>
  
  <p>StackTrace</p>
  
  <pre><code>/home/abid/.bundler/ruby/1.9.1/nested_form-6232dd853c27/lib/nested_form/builder_mixin.rb:41:in `block in link_to_add'
  /home/abid/.bundler/ruby/1.9.1/nested_form-6232dd853c27/lib/nested_form/view_helper.rb:53:in `call'
  /home/abid/.bundler/ruby/1.9.1/nested_form-6232dd853c27/lib/nested_form/view_helper.rb:53:in `after_nested_form_callbacks'
  /home/abid/.bundler/ruby/1.9.1/nested_form-6232dd853c27/lib/nested_form/view_helper.rb:8:in `block in nested_form_for'
  </code></pre>
  
  <p>Any Ideas ?</p>