checked check_box set from stash or param.

gistfile1.pl

use Test::More tests => 3;
use Test::Mojo;

{
  package Foo;
  use Mojo::Base 'Mojolicious';

  sub startup {
    my $self = shift;
    my $r = $self->routes;
    $r->get('form/:test')->to( 'Form#form' );
  }

  1;
}

{
  package Foo::Form;
  use Mojo::Base 'Mojolicious::Controller';

  sub form {
    my ($self, $args) = @_;

    $self->stash(foo => 'baz');
    $self->param(a => 'a');
    $self->stash(bar => 'foo');
    $self->render(template => 'form');
  }

}

my $t = Test::Mojo->new('Foo');
$t->get_ok('/form/23')->status_is(200)->content_is(<<EOF);
<form action="form" method="post">
  <input name="foo" value="baz" />
  <input name="bar" type="checkbox" checked="checked" value="1" />
  <input name="a" type="checkbox" checked="checked" value="2" />
  <input type="submit" value="Ok!" />
  <input id="bar" type="submit" value="Ok too!" />
</form>
EOF

__DATA__
@@ form.html.ep
%= form_for 'form', {test => 24}, method => 'post' => begin
  %= text_field 'foo', value => stash 'foo'
  %= check_box bar => 1
  %= check_box a => 2
  %= submit_button 'Ok!'
  %= submit_button 'Ok too!', id => 'bar'
%= end