public void bindTo(Object proxy) {
    if (handle != null) {
      throw new IllegalStateException(
          "Attempted to rebind a default method handler that was already bound");
    }
    handle = unboundHandle.bindTo(proxy);
  }
Bind this handler to a proxy object. After bound, DefaultMethodHandler#invoke will act as if it
was called on the proxy object. Must be called once and only once for a given instance of
DefaultMethodHandler
core/src/main/java/feign/DefaultMethodHandler.java