allow custom object nodes

Author: marschall

src/main/java/com/fasterxml/jackson/databind/node/NodeCursor.java

@@ -195,7 +195,10 @@ protected final static class ObjectCursor
         public ObjectCursor(JsonNode n, NodeCursor p)
         {
             super(JsonStreamContext.TYPE_OBJECT, p);
-            _contents = ((ObjectNode) n).fields();
+            if (!n.isObject()) {
+                throw new IllegalStateException("Current node is not an object node");
+            }
+            _contents = n.fields();
             _needEntry = true;
         }