array('hook') syntax for suggestions is broken for #theme_wrappers

This is partially my fault in #2042285: #theme_wrappers should be able to have a unique set of variables per wrapper hook we broke #theme_wrappers that use array('hook') syntax.

Eg:

$foo = array(
  '#theme_wrappers' => array(array('foo')),
);

No longer works as drupal_render() now assumes that array values = attribute overrides.