Fix 'shape-inside' added to <tspan> when saving

Prevent that style="shape-inside:url(#...)" is added to fallback <tspan> elements.

Example:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<svg xmlns="http://www.w3.org/2000/svg">
  <ellipse id="x" style="fill:none;stroke:#000000" cx="50" cy="50" rx="40" ry="40" />
  <text style="shape-inside:url(#x)">Hello</text>
</svg>

Without this patch, re-saving this file produces:

  <text style="shape-inside:url(#x)"><tspan
       x="47.735306"
       y="46.574219"><tspan
         style="shape-inside:url(#x)">Hello</tspan></tspan></text>

@Tavmjong - Question about the code I changed: Under which circumstances will style from source_obj need to be written to such tspans?