title: Why is the option not selected the second time in multi select list?
body: <p>I need to select deselect options in multi-select via JavaScript. The code seems alright but it does not fire the second time you try to select the same item.</p>
  
  <p>So try selecting <strong>Full-Time</strong>, again click it, that will be deselected. Try clicking it again and it won't work.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>$('.multi-select option').on('mousedown',function (e) {
      e.preventDefault();
  
      selectTop = $(this).parent().scrollTop(); 
      var before = $(this).prop("selected");
      console.warn("before: " + before);
      if (before == true) {
          $(this).prop("selected", false);
      }
      else {
          $(this).attr("selected", "selected");
      }
      console.info("Changed from " + before + " to " + $(this).prop('selected')); 
      mustChangeScrollTop = true;
  
      return true;
  });</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"&gt;&lt;/script&gt;
  &lt;html&gt;
    
    &lt;head&gt;
      &lt;title&gt;Multi select jquery test&lt;/title&gt;
    &lt;/head&gt;
    
    &lt;body&gt;
      &lt;select size="6" name="WorkSchedule" multiple="multiple" id="WorkSchedule" class="form-control multi-select"&gt;
     &lt;option value=""&gt;Select All That Apply&lt;/option&gt;
     &lt;option value="4"&gt;Full-Time&lt;/option&gt;
     &lt;option value="5" &gt;Part-Time&lt;/option&gt;
     &lt;option value="6" &gt;Per Diem&lt;/option&gt;
     &lt;option value="7"&gt;Locum Tenens&lt;/option&gt;
    &lt;/select&gt;
    &lt;/body&gt;
  
  &lt;/html&gt;</code></pre>
  </div>
  </div>
  </p>