fix a bug in svgToString function: some elements have no attribute, so when convert it to stinrg some namespace can't be set correctly.
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>
yushiro/svg-edit,eueung/svg-edit-mirror,kahunamoore/svg-edit,eueung/svg-edit-mirror,yushiro/svg-edit,kahunamoore/svg-edit,yushiro/svg-edit,kahunamoore/svg-edit,eueung/svg-edit-mirror,yushiro/svg-edit,eueung/svg-edit-mirror,kahunamoore/svg-edit