XmlUtil.serialize: better pretty printing

Change-Id: Ibe1e51e670e664a120024cc903ffcf4c80d2e00f
Reviewed-on: http://gerrit.esdi-community.eu/3128
Tested-by: Jenkins <sitemple@igd.fraunhofer.de>
Reviewed-by: Simon Templer <simon.templer@igd.fraunhofer.de>