Save as optimized produces an error

[Martijn]

When I save as Optimized SVG, I get this error:

```
Traceback (most recent call last):
  File "scour.inkscape.py", line 78, in <module>
    e.affect(output=False)
  File "C:\Program Files\Inkscape\share\extensions\inkex.py", line 265, in affect
    self.getposinlayer()
  File "C:\Program Files\Inkscape\share\extensions\inkex.py", line 207, in getposinlayer
    x = self.unittouu( xattr[0] + 'px' )
  File "C:\Program Files\Inkscape\share\extensions\inkex.py", line 351, in unittouu
    return retval * (self.__uuconv[u.string[u.start():u.end()]] / self.__uuconv[self.getDocumentUnit()])
  File "C:\Program Files\Inkscape\share\extensions\inkex.py", line 304, in getDocumentUnit
    p = param.match(svgwidth)
TypeError: expected string or buffer
```

This is on Inkscape 0.91 r13725 on Windows 7 x64.

SVG that this happens with will be attached.

[su_v (suv-lp)]

Thank you for taking the time to file this report - the issue has been reported before and is tracked in
* Bug #1463623 “Error trying to save Optimized SVG (no 'width' attribute for SVGRoot)”
  https://bugs.launchpad.net/inkscape/+bug/1463623

Linking as duplicate to bug #1463623.