Fix possible ClassCastException, because DSet.contains(E entry)

will type-erase to DSet.Entry. Ensure objects tested for containment
in our Set view implement that.


git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@6128 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/java/com/threerings/presents/dobj/DSet.java

@@ -272,6 +272,9 @@ public class DSet<E extends DSet.Entry>
                 throw new UnsupportedOperationException();
             }
             @Override public boolean contains (Object o) {
+                if (!(o instanceof DSet.Entry)) {
+                    return false;
+                }
                 @SuppressWarnings("unchecked") E elem = (E)o;
                 return DSet.this.contains(elem);
             }