title: QDateTime fromString formate have bug
body: <p>The Qt version which I use is 5.4. When I try this code blow, the result is OK</p>
  
  <pre><code>QString dateString = "Mar 4, 2015";
  QDateTime dateData = QDateTime::fromString(dateString, "MMM dd, yyyy");
  qDebug() &lt;&lt; "data:" &lt;&lt; dateData.toString("yyyy dd MM");
  </code></pre>
  
  <p>the result is data: <code>2015 04 03</code>.
  But when I change the string to <code>Mar 3, 2015</code>, the result is "". Why?</p>
  
  <pre><code>QString dateString = "Mar 3, 2015";
  QDateTime dateData = QDateTime::fromString(dateString, "MMM dd, yyyy");
  qDebug() &lt;&lt; "data:" &lt;&lt; dateData.toString("yyyy dd MM");
  </code></pre>