useForm returns undecorated form at first render

### Are you submitting a **bug report** or a **feature request**?

Bug

### What is the current behavior?

Decorators are added ofter form is rendered.

### What is the expected behavior?

Decorators should be added before render.

### Sandbox Link
https://codesandbox.io/s/silly-ritchie-pn9ns

### What's your environment?
RFF v6.3.0

### Other information

My custom decorator adds a method on the form object (which registers event listeners) but in a component that calls `useForm` the method isn't available on the form object on first render.