(Bug 62663) Temp fix for handling non-string extension attribute values

* This patch forcibly converts non-string extension attribute values
  to string and passes them along. On RT-ing, this does introduce
  dirty diffs (&nbsp; --> " "), but selser should cover most of these
  uses.

  This eliminates the crasher reported in the bug report.

Change-Id: I765c5d7c55503681ce21c92a20a4db53569bcd4e