Support exportSVG for treePath nodes

fix error on try export textPath like this
```html
<text y="-1312.796875" x="213" font-family="Helvetica, Arial, sans-serif" id="SvgjsText1023">
    <textPath xlink:href="#SvgjsPath1026" id="SvgjsTextPath1025">
        <tspan dy="-10" id="SvgjsTspan1024">Long description of link ssw1 <--> ssw2</tspan>
    </textPath>
</text>
```