Fixed '<g> </g>' becoming '<g></g>' and made beautification idempotent

[alubbe]

Both fixes come with tests

Idempotence means that applying beautification more than once does not change the result. Before, running it twice on the test input yielded this

  <d>
    <soapenv:Envelope

      xmlns:soapenv="http://xxx"

      xmlns:xsd="http://yyy"

      xmlns:xsi="http://zzz">
    </soapenv>
  </d>

[alubbe]

hey @vkiryukhin is this package still maintained / is there any interest in seeing this PR merged? If you prefer, I can clean up the diff by extracting the whitespace commits into a separate PR. Happy 2017!