formatOnBlur is also updating the value in the form state

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

### What is the current behavior?
When formatOnBlur is used the value of that field is also updated in the form state in the format which is passed to field.

### What is the expected behavior?

It should just format the value to display in the field, should not update on form state

### What's your environment?

react-final-form: 3.6.5
Chrome browser
node: v10.5.0

### Other information

https://codesandbox.io/s/zwk325kl
type any number in the price field and see the form values on blur of the field