If multiple is true, return meta as array

When custom field type multiple argument is specifically set to true, the data is saved properly, I believe this was done by pull #98, but when showing a custom field, meta is still returned as single key, since it still  only checks

    $field['multiple'] = 'multicheck' == $field['type'];

and not if multiple was already set. This pull adds the same check as pull #98 but on the front-end part show_form, so that meta is returned as array if multiple is true.