title: read_xml() throw exception due to lacking of double quotes
body: <p>I use boost::property_tree object to parse xml like this:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;root&gt;
      &lt;node attr="attr_str"/&gt;
  &lt;/root&gt;
  </code></pre>
  
  <p>When I call read_xml() to parse this content, it works well. But if I remove those double quotes around the <strong>attr</strong> attribute like this:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;root&gt;
      &lt;node attr=attr_str/&gt;
  &lt;/root&gt;
  </code></pre>
  
  <p>It throw the <strong>xml_parse_error</strong> exception.</p>
  
  <p>Does any flags can be set to ignore the checking of double quotes?</p>