title: select_tag helper with grouped_options_for_select order
body: <p>I have this in my model:</p>
  
  <pre><code>LOCATION_IN_UK = {'England' =&gt; [['Berkshire', 1],['Cambridgeshire',2],['Cheshire',3]], 'Scotland' =&gt; [['Dumfries and Galloway',4],['Fife',5],['Lothian',6]], 'Others' =&gt; [['Outside UK',7]]}
  </code></pre>
  
  <p>And this is in the view:</p>
  
  <pre><code>&lt;%= select_tag :location, grouped_options_for_select(Location::LOCATION_IN_UK), :id =&gt; 'location-dropdown' %&gt;
  </code></pre>
  
  <p>This code generate following html:</p>
  
  <pre><code>&lt;select id="location-dropdown" name="location"&gt;
    &lt;optgroup label="England"&gt;
      &lt;option value="1"&gt;Berkshire&lt;/option&gt; 
      &lt;option value="2"&gt;Cambridgeshire&lt;/option&gt; 
      &lt;option value="3"&gt;Cheshire&lt;/option&gt;&lt;/optgroup&gt;
    &lt;optgroup label="Others"&gt;
      &lt;option value="7"&gt;Outside UK&lt;/option&gt;&lt;/optgroup&gt;
    &lt;optgroup label="Scotland"&gt;
      &lt;option value="4"&gt;Dumfries and Galloway&lt;/option&gt; 
      &lt;option value="5"&gt;Fife&lt;/option&gt; 
      &lt;option value="6"&gt;Lothian&lt;/option&gt;&lt;/optgroup&gt;
  &lt;/select&gt; 
  </code></pre>
  
  <p><strong>1.</strong> How to skip alphabetical sort order. I want elements locate exactly as in the hash LOCATION_IN_UK.<br>
  <strong>2.</strong> How to insert prompt into this? <code>:prompt =&gt; 'Please select'</code> Doesn't work</p>