_product_specials hook clobbers other discounts

The _product_specials hook ignores other specials that may be set by other modules.

Instead of returning a new array like return array('special' => $discounts); it should add an entry to the $specials array argument that was passed into the function and return the result. Also, instead of using a generic key like 'special', which can collide with keys set by other modules, use a more specific key:

$specials['quantity discount'] = $discounts;

return $specials;

The Role Discount module for example will definitely conflict with Quantity Discount. Whichever happens to be processed last will overwrite the discount set by the other (Role Discount also has this very same bug).