Get rid of Java generics warning
Get rid of Java generics warning

Casts are still needed, because Properties extends
Hashtable<Object, Object> instead of Hashtable<String, String>.

davidmoten/UMLGraph,Edd123/UMLGraph,dspinellis/UMLGraph,Edd123/UMLGraph,Edd123/UMLGraph,davidmoten/UMLGraph,davidmoten/UMLGraph,dspinellis/UMLGraph,dspinellis/UMLGraph