@Override
  public Object invoke(Object[] argv) throws Throwable {
    if (handle == null) {
      throw new IllegalStateException(
          "Default method handler invoked before proxy has been bound.");
    }
    return handle.invokeWithArguments(argv);
  }
Invoke this method. DefaultMethodHandler#bindTo must be called before the first time invoke is
called.
core/src/main/java/feign/DefaultMethodHandler.java