title: xmlstarlet update an attribute
body: <p>I'm working on a script which deals with an xml file.
  I'd like to update an attribute value in this xml file with xmlstarlet but it doesn't work.</p>
  
  <p>Here is a sample of the xml file:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;channel name="xfce4-keyboard-shortcuts" version="1.0"&gt;
    &lt;property name="commands" type="empty"&gt;
      &lt;property name="default" type="empty"&gt;
        &lt;property name="&amp;lt;Alt&amp;gt;F2" type="empty"/&gt;
        &lt;property name="&amp;lt;Control&amp;gt;&amp;lt;Alt&amp;gt;Delete" type="empty"/&gt;
        &lt;property name="XF86Display" type="string" value="xrandr --output LVDS --auto --output VGA-0 --mode 1680x1050_60.00 --right-of LVDS"/&gt;
      &lt;/property&gt;
      &lt;property name="custom" type="empty"&gt;
        &lt;property name="&amp;lt;Alt&amp;gt;F2" type="string" value="xfrun4"/&gt;
        &lt;property name="&amp;lt;Control&amp;gt;&amp;lt;Alt&amp;gt;Delete" type="string" value="xflock4"/&gt;
        &lt;property name="XF86Display" type="string" value="xrandr --output LVDS --auto --output VGA-0 --mode 1680x1050_60.00 --right-of LVDS"/&gt;
        &lt;property name="override" type="bool" value="true"/&gt;
      &lt;/property&gt;
    &lt;/property&gt;
  &lt;/channel&gt;
  </code></pre>
  
  <p>And here is the command to update the property with the name "XF86Display" in the custom property node.</p>
  
  <pre><code>xmlstarlet edit \
    --update "/xml/channel[@name=xfce4-keyboard-shortcuts]/property[@name=commands]/property[@name=custom]/property[@name=XF86Display]/@value" \
    --value "test" xfce4-keyboard-shortcuts.xml
  </code></pre>
  
  <p>This output is strictly the same to the input...</p>
  
  <p>Thank you</p>