make sure the fixup for key() reported by John Escott actually works.

* libxslt/functions.c: make sure the fixup for key() reported
  by John Escott actually works.
* tests/docs/Makefile.am tests/docs/bug-98.xml
  tests/general/Makefile.am tests/general/bug-98.*: added the
  example in the regression tests for this case
Daniel