Type inference broken in 6.5.1?

Hi,

I just updated from 6.4.0 to 6.5.1 and got some TS errors due to #806

Here is a sample example:

```
{ ... }}>
```

In 6.4.0, `values` argument of the onSubmit function is of type `{ name:string; }`. In 6.5.1, it is a `Record`.

Looking a bit at the type definition, it looks like 6.4 is able to infer type of `values` from `initialValues` while 6.5 cannot.

I also updated final-form from 4.19.1 to 4.20.1 though I'm not sure that's relevant.

Thanks