title: Set id of select options to jstl variable
body: <p>I've a JSP page that load some info from a DAO to a select element.
  Something like this:</p>
  
  <pre><code>&lt;select id="dropdown" onchange="changeKa()"&gt;
    &lt;c:forEach var="ka" items="${KaList}"&gt;
      &lt;option value="&lt;c:out value='${ka.area}' /&gt;" 
        &lt;c:if test="${param.selectValue == ka.area})"&gt; selected &lt;/c:if&gt;  &gt;
        &lt;c:out value="${ka.area}" /&gt;
      &lt;/option&gt;
    &lt;/c:forEach&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>Where I have</p>
  
  <pre><code>&lt;option value="&lt;c:out value='${ka.area}' /&gt;" ... 
  </code></pre>
  
  <p>I want to set an id like this:</p>
  
  <pre><code>&lt;option value="&lt;c:out value='${ka.area}' /&gt;" id="&lt;c:out value='${ka.id_knowledgearea}' /&gt;" ...
  </code></pre>
  
  <p>But the following error appears:</p>
  
  <pre><code>Bad value "   " for attribute "id" on element "option": An ID must not contain whitespace.
  </code></pre>
  
  <p>How can I solve this?</p>