`render_kw` for options are not passed on correctly to `_Option` in `SelectFieldBase.__iter__`

[Daverball]

wtforms/src/wtforms/fields/choices.py

Lines 59 to 61 in 28f4e1f

	opt = self._Option(
	label=label, id="%s-%d" % (self.id, i), **opts, **render_kw
	)

Instead of merging render_kw with opts['render_kw'] they're directly used as arguments for the creation of the _Option instance. This code path currently doesn't appear to be exercised in the unit tests. You can easily reproduce this bug by adding a unit test to https://github.com/pallets-eco/wtforms/blob/main/tests/fields/test_radio.py that tries to pass in three-tuple choices instead of two-tuple choices.

Here's the SelectField test case adapted for RadioField:

def test_option_render_kw():
    F = make_form(
        a=RadioField(choices=[("a", "Foo", {"title": "foobar", "data-foo": "bar"})])
    )
    form = F(a="a")
    assert form.a() == (
        '<ul id="a">'
        '<li><input checked id="a-0" name="a" type="radio" '
        'value="a" title="foobar" data-foo="bar"> '
        '<label for="a-0">yes</label></li></ul>'
    )

The fix should be fairly simple, something like the following should work perfectly fine:

    def __iter__(self):
        opts = dict(
            widget=self.option_widget,
            validators=self.validators,
            name=self.name,
            _form=None,
            _meta=self.meta,
        )
        for i, choice in enumerate(self.iter_choices()):
            if len(choice) == 4:
                value, label, checked, render_kw = choice
                if render_kw and self.render_kw:
                    render_kw = self.render_kw | render_kw
                elif self.render_kw:
                    render_kw = self.render_kw
            else:
                value, label, checked = choice
                render_kw = self.render_kw

            opt = self._Option(
                label=label, id="%s-%d" % (self.id, i), render_kw=render_kw, **opts, 
            )
            opt.process(None, value)
            opt.checked = checked
            yield opt