fix(radio-button-group): set name attribute on inputs

[metonym]

Fixes #1814, partially addresses #1813

Adds a name prop to RadioButtonGroup. If provided, name is set on the rendered input elements.

<RadioButtonGroup name="choices">
  <RadioButton id="1" value="1" />
  <RadioButton  id="2" value="2" />
</RadioButtonGroup>

<!-- DOM representation (abridged) -->
<fieldset>
  <input name="choices"  id="1" value="1" />
  <input name="choices"  id="2" value="2" />
</fieldset>

[brunnerh]

Have you seen #1037?

[metonym]

@brunnerh I haven't. Let's move to yours.