title: SimpleXML::addChild() can't add line break when output to a xml
body: <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;&lt;items&gt;
  &lt;item&gt;&lt;title&gt;title3&lt;/title&gt;&lt;desc&gt;This is some desc3&lt;/desc&gt;&lt;/item&gt;&lt;/items&gt;
  </code></pre>
  
  <p>There is no line break between each node element when using asXML() to output?</p>
  
  <p>How to make output the file well-structured by adding a line break after each XML elements opening and closing tag that contains child element nodes:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;items&gt;
  &lt;item&gt;
  &lt;title&gt;title3&lt;/title&gt;
  &lt;desc&gt;This is some desc3&lt;/desc&gt;
  &lt;/item&gt;
  &lt;/items&gt;
  </code></pre>