title: Select with :multiple => true not saving values
body: <p>Im trying to save multiple selected values form a multiple select field generated by the rails helper select.</p>
  
  <pre><code>  &lt;div class="form-group"&gt;
      &lt;%= f.label :available_type, "Available in category" %&gt;&lt;br&gt;
      &lt;%= f.select :available_type, options_for_select(Setting.subscription_type, @terminal.available_type), { }, { class: "form-control", :multiple =&gt; true, :size =&gt; 5    } %&gt;
    &lt;/div&gt;
  </code></pre>
  
  <p>This renders like this (the selected value is from a previous attempt without the ":multiple => true" attribute that works perfectly): </p>
  
  <pre><code>&lt;select class="form-control" id="terminal_available_type" multiple="multiple" name="terminal[available_type][]" size="5"&gt;
  &lt;option value="Postpaid"&gt;Postpaid&lt;/option&gt;
  &lt;option value="MBB"&gt;MBB&lt;/option&gt;
  &lt;option selected="selected" value="Prepaid"&gt;Prepaid&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/hoMNa.png" alt="enter image description here"></p>
  
  <p>Any help is appreciated. :)</p>
  
  <p><strong>Edit:</strong>
  I have tried putting <code>serialize :available_type</code> in my Terminal model, dident change anything. :-/</p>
  
  <p><strong>Edit 2:</strong>
  I noticed that the multiple selcted field does not mark options as selected when i mark them. if i add Selected attributes manualy i get these parametres:</p>
  
  <pre><code>{"utf8"=&gt;"✓", "_method"=&gt;"patch", "authenticity_token"=&gt;"RrwWlKk8XlGeC+dTu/w6oSM68e9LcbUFJWTI+eRS9mI=", "terminal"=&gt;{"inndate"=&gt;"2015-01-13", "outdate"=&gt;"", "brand_id"=&gt;"2", "name"=&gt;"iPhone 5c", "available_type"=&gt;["", "MBB", "Prepaid"], "product_number"=&gt;"3r2342", "ean_code"=&gt;"", "navision_nb"=&gt;"324234", "cost_price_map"=&gt;"3200.0", "manual_price"=&gt;"", "sales_info"=&gt;"Just sell!"}, "commit"=&gt;"Submit", "action"=&gt;"update", "controller"=&gt;"terminals", "id"=&gt;"2"}
  </code></pre>
  
  <p>The available_type field has the value  <code>"available_type"=&gt;["", "MBB", "Prepaid"]</code></p>
  
  <p>Im using rails 4.0.2 and here are my strong parametres:</p>
  
  <pre><code># Never trust parameters from the scary internet, only allow the white list through.
  def terminal_params
    params.require(:terminal).permit(:inndate, :outdate, :brand_id, :name, :product_number, :navision_nb, :cost_price_map, :manual_price, :sales_info, :available_type)
  end
  </code></pre>