check for the correct field type

HTML allow to define different form fields with the same name.
Imagine the following form:

    <html>
        <body>
            <form action="/">
                <input type="hidden" name="option" value="default">
                <input type="radio" name="option" value="A">
                <input type="radio" name="option" value="B">
                <input type="hidden" name="settings[1]" value="0">
                <input type="checkbox" name="settings[1]" value="1" id="setting-1">
                <button>klickme</button>
            </form>
        </body>
    </html>

Since the `FormFieldRegistry` can only handle one field per name, the
hidden field option is registered first before the radio field with
the same name is evaluated. Thus, the `FormFieldRegistry` returns an
`InputFormField` instance on which the `addChoices()` method can not
be called.