enhanced previous fix to bug #120684, using excellent suggestion by Daniel

* transform.c: enhanced previous fix to bug #120684, using
  excellent suggestion by Daniel
* attributes.c: fixed bug #119583, merging attribute sets
  from imported stylesheets.
* tests/docs/Makefile.am tests/docs/bug-131.xml
  tests/general/Makefile.am tests/docs/bug-131*: added tests
  to the regression suite for bug #120684.