Initial value on select is not synced

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

Bug report

### What is the current behavior?

Given a field with a `` input, browsers will pre-select the first option. If initialValues is not provided, this pre-selected value will not be synchronized with final-form's internal state. Since this is not obvious, it quickly leads to bugs. Of course it can be circumvented using `initialValues`, but that's not obvious and a bit tedious (especially with an asynchronous list of options).

### What is the expected behavior?

The first (initial) option of a `` should be automatically picked up by react-final-form and synced with final-form's internal state. At the very least there should be a warning when using `` without providing a value for it through `initialValues`.

### Sandbox Link

https://codesandbox.io/s/8ymv5on2wl

Note that 'red' is not persisted in final-form's state, despite it being initially selected by the browser. The only way to make it work is to first select another option, then select red again.