Preserve theme-wrappers on forms.

Hi,

I had a form that had other theme-wrappers besides 'form', but these were stripped by bundling them together in a Multiform.

This is because Multiform simply does $current_form['#theme_wrappers'] = array('container');, instead of just replacing the 'form'-wrapper with the 'container'-wrapper.

Attached is a patch that uses array_search to perserve non-'form' theme-wrappers, while still changing the 'form' theme-wrapper to 'container'.

Kind regards
Morten