fixing bug #134500 on namespace lookup for attribute which sometimes lead

* libxslt/namespaces.c libxslt/namespaces.h libxslt/templates.c
  libxslt/transform.c: fixing bug #134500 on namespace lookup for
  attribute which sometimes lead to default namespace
* tests/general/bug-150*, tests/docs/bug-150.xml,
  tests/general/Makefile.am, tests/docs/Makefile.am: added
  regression test for bug #134500
Daniel