* libexslt/math.c, libexslt/common.c: fixed problem,
  reported on the list by Markus Bayerlein, concerning
  math functions on nodesets generated with
  exslt:node-set
* tests/exslt/math/max.3.xsl, tests/exslt/math/max.3.xml,
  tests/exslt/math/max.3.out, tests/exslt/math/Makefile.am:
  added test case for above.