Fix varargs (i.e. <n+>) signature validation. #48

Was only working correct for arrays <a+> signatures

Author: uw4

src/main/java/com/dashjoin/jsonata/utils/Signature.java

@@ -385,6 +385,7 @@ public Object validate(Object _args, Object context) {
                             validatedArgs.add(arg);
                             argIndex++;
                         } else {
+                            arg = argIndex<args.size() ? args.get(argIndex) : null;
                             validatedArgs.add(arg);
                             argIndex++;
                         }