is_dropdown can evaluate to true even if depth is not specified in wp_nav_menu() arguments

is_dropdown can evaluate to true even if max_depth is 0. This is the WordPress default and implies that dropdowns are enabled. The current code seems to interperet $max_depth == 0 to mean dropdowns are disabled.

http://codex.wordpress.org/Function_Reference/wp_nav_menu :
<blockquote>$depth
(integer) (optional) How many levels of the hierarchy are to be included <strong>where 0 means all</strong>.</blockquote>