title: Sabre XML writer's output is a plain string
body: <p>I am using the Sabre XML writer library.</p>
  
  <p>I am trying the example from the site itself:</p>
  
  <pre><code> $writer = new Sabre\Xml\Writer();
          $writer-&gt;openMemory();
          $writer-&gt;namespaceMap = [
              'http://example.org/' =&gt; 'e',
          ];
  
          $writer-&gt;startElement('{http://example.org/}root');
          $writer-&gt;write([
      '{http://example.org/ns}title' =&gt; 'Foundation',
      '{http://example.org/ns}author' =&gt; 'Isaac Asimov',]);
  
          $writer-&gt;endElement();
          echo $writer-&gt;outputMemory();
  </code></pre>
  
  <p>I am expecting this output:</p>
  
  <pre><code>&lt;?xml version="1.0"&gt;
  &lt;e:root&gt;
      &lt;e:title&gt;Foundation&lt;/e:title&gt;
      &lt;e:author&gt;Isaac Asimov&lt;/e:author&gt;
  &lt;/e:root&gt;
  </code></pre>
  
  <p>but instead I got a plain string:</p>
  
  <pre><code>FoundationIsaac Asimov
  </code></pre>
  
  <p>I have  tried the XML reader functions and they work just fine, so the package is correctly instealled.</p>
  
  <p>Any help?</p>