Prevent AttributeError unicode object no toString
Prevent AttributeError unicode object no toString

This patch is a fix for the following error:

`AttributeError: 'unicode' object has no attribute 'toString'`

Note: The error might be related to QGIS version 2.x only, not 1.x. The
effect of this patch on the plugin under QGIS 1.x has not been tested!

rkistner/chinese-postman,rkistner/chinese-postman