added some logic to xsltKeyFunction to set the transform context doc ptr

* libxslt/functions.c: added some logic to xsltKeyFunction
  to set the transform context doc ptr when the xpath context
  doc is different (bug 147445)
* tests/exslt/common/Makefile.am, tests/exslt/common/node-set.5*
  and tests/exslt/common/node-set.6*: added regression tests
  for bug 145547 (UNION and FILTER for RVT's)