[xml] pretty-print & format

[Afanasiy Fet <dailylama gmail com>]

While libxml does a great job reducing amount of text with ‘xmlDocDumpFormatMemoryEnc’ 's last argument, ‘format’ to remove spaces & newlines and converting <Node></Node> to <Node/>.

I'd like it to drop the root node completely if all childs are empty, like here:

<root>

  <Node1/>

</root>

Or something else what may help reduce network load.