better fix for handling of global variables in XSLs

This fixes Xamarin bug 18118. Added unit test.

Change-Id: I8da42cd8641bfa0eb7029af286938edbaa0599bc
added unit tests for XSL node-set bug

Also, fixed existing unit tests for XslTransform

See Xamarin bug 18114

Change-Id: I0bd05148b6cbde43632befb0f23b995ee99ad68f
added unit test for XSL value-of bug

See Xamarin bug 18113

Change-Id: I634113a7dbc4dca82588e98d7c50e57f30da0131