code: public void invoke(String methodName, Object argument, JsonRpcCallback<Object> callback) {
  		invoke(methodName, argument, Object.class, new HashMap<String, String>(), callback);
  	}
docs: Invokes the given method with the given arguments and invokes the
  {@code JsonRpcCallback} with the result.
  
  @param methodName the name of the method to invoke
  @param argument   the arguments to the method
  @param callback   the {@code JsonRpcCallback}