fix #8546 - Validator do not detect errors named identically (partial revert of r5783, reopens #8517). This is due to TreeMap that uses compareTo() instead of equals() (see Javadoc)

git-svn-id: http://josm.openstreetmap.de/svn/trunk@5822 0c6e7542-c601-0410-84e7-c038aed88b3b