Fix conflict resolution for templates with same priority

The spec states in "5.5 Conflict Resolution for Template Rules":

> It is an error if this leaves more than one matching template rule.
> An XSLT processor may signal the error; if it does not signal the
> error, it must recover by choosing, from amongst the matching
> template rules that are left, the one that occurs last in the
> stylesheet.

We don't signal an error, so we must make sure to return the last
template in the stylesheet.

Fixes #55