Database Forge Class - Set Default Collation

Is there a way to set the default collation using Codeigniter's Database Forge Class? It defaults to 'utf8_general_ci' but I want to override that to 'utf8_unicode_ci'.

Code:

$attributes = array(
 'ENGINE' => 'InnoDB',
 'COLLATION' => 'utf8_unicode_ci'
);
$this->dbforge->create_table(table_name, FALSE, $attributes);

When I add this (my best guess on how to set it) it produces an error.

Code:

'COLLATION' => 'utf8_unicode_ci'

Documentation does not specify how to set the default collation.