theme system cannot override *_node_form forms

How to produce the bug:
Register a template for a *_node_form in the theme hook_theme()
e.g.

mytheme_theme() {
    return array(
    'poll_node_form' => array(
      'template'  => 'poll-node-form',
      'arguments' => array('form' => NULL),
    ),
  );
}

Expected behaviour:
Theme system should recognize this newly registered form theming.

What happens instead:
drupal_render_form sees that node_form has specified #theme - and doesn't bother to check if any module or theme has registered its own theme for that form.

from drupal_render_form

// Don't override #theme if someone already set it.

Solution:
Remove the #theme item from the node_form form.