Form InitialValues overrides Field InitialValue

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

**bug report**

### What is the current behavior?

- I have some Form initialValues fetched on component did mount. Initial State is empty array.
- I have some Fields initialValue set for each of my form fields.
- Before the fetch of of my Form initialValues the Field initialValue are correct.
- After fetch has completed the initialValues of the Field are set to overwritten with Form InitialValues.

### What is the expected behavior?

- Field InitialValue should never be overwritten by the Form InitialValues.

### What's your environment?

"final-form": "^4.12.0",
"react-final-form": "^5.0.1",

### Example
https://codesandbox.io/embed/react-final-form-simple-example-qb57e?fontsize=14