fix a bug in svgToString function:
some elements have no attribute, so when convert it to stinrg some namespace can't be set correctly.

now, the tag like this can be correctly converted:
<se:mytag>hello</se:mytag>

git-svn-id: http://svg-edit.googlecode.com/svn/trunk@2517 eee81c28-f429-11dd-99c0-75d572ba1ddd