title: Select with attribute not in model without attr_accessor
body: <p>I'm trying to attach some data to my form not directly linked to my model.</p>
  
  <p>I can do it with a <code>radio_button</code> but not at all with a <code>select</code> (<code>collection_select</code> etc).</p>
  
  <p>Here is my code:</p>
  
  <pre><code>= form_for @user, url: user_path(@user) do |f| 
    // This is working
    = f.radio_button :my_field_not_in_model, true, checked: true
      | Yes
    = f.radio_button : my_field_not_in_model, false, checked: false
      | No
    br
    // This is not working
    = f.label 'Another user'
    = f.collection_select :reference_user_id, @user.group.users, :id, :email
  </code></pre>
  
  <p>Basically, I would like to attach a reference user to make a special analysis on my controller before removing this attribute before the final update. I don't want to create an <code>attr_accessor</code> on my model each time I need more data for a special treatment on my controller.</p>