Double-incrementing number fields

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

### What is the current behavior?
Clicking the increment buttons on an `` results in behaviour like that of `step=2`.

### What is the expected behavior?
It should behave like `step=1`.

### Sandbox Link
https://codesandbox.io/s/double-incrementing-with-react-final-form-rp2so

### What's your environment?
final-form `4.18.5`
react `16.8.6`
react-dom `16.8.6`
react-final-form `6.3.0`
react-scripts `3.0.1`

Chrome `77.0.3865.120`
macOS `10.15`

### Other information