Author mapping not sanitized

We do not sanitize the $old_login when setting it as a key to $this->author_mapping in get_author_mapping().

When we goto reference it in process_posts() we do this:

// map the post author
$author = sanitize_user( $post['post_author'], true );
if ( isset( $this->author_mapping[$author] ) )
	$author = $this->author_mapping[$author];
else
	$author = (int) get_current_user_id();

The sanitize_user call will remove multiple characters (including white spaces) and replace uppercase w/ lowercase causing the key to not be found.