title: Show concatenated text for collections_select option text
body: <p>I have a collections_select:</p>
  
  <p><code>collection_select(:building, :room_id, Room.all, :id, :name, prompt: true)</code>:</p>
  
  <pre><code>&lt;select name="building[room_id]"&gt;
    &lt;option value=""&gt;Please select&lt;/option&gt;
    &lt;option value="1" selected="selected"&gt;Room A&lt;/option&gt;
    &lt;option value="2"&gt;Room B&lt;/option&gt;
    &lt;option value="3"&gt;Room C&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>I would like to have <code>:name</code> and <code>:number</code> as the text for the options tag:</p>
  
  <pre><code>&lt;select name="building[room_id]"&gt;
    &lt;option value=""&gt;Please select&lt;/option&gt;
    &lt;option value="1" selected="selected"&gt;Room A - 101&lt;/option&gt;
    &lt;option value="2"&gt;Room B - 102&lt;/option&gt;
    &lt;option value="3"&gt;Room C -103&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>