Field.Group: Support "exact" prop

[kettanaito]

What

I suggest to add a new exact prop to the <Field.Group/> component.

Why

To prevent groups closing/reopening for nested group that must belong to the group outside.

Example

<Field.Group name="first">
  <Field.Group name="second">
    <Input name="fieldOne" initialValue="foo" />
  </Field.Group>

  <Field.Group exact name="second">
    <Input name="fieldOne" initialValue="bar" />
  </Field.Group>
</Field.Group>

Serializes into:

{
  "first": {
    "second": {
      "fieldOne": "foo"
    }
  },
  "second": { // exact
    "fieldOne": "bar"
  }
}

[kettanaito]

The support for exact prop is added. The integration tests for Field.Group have been re-written and simplified.