Product to be removed from cart when update count is equal or less than 0

Problem/Motivation

Currently if we update cart with 0 count, then item is removed from cart. I have not tested with less than zero count but I think it may break the functionality.

Proposed resolution

Handle less than 0 update count. If 0 or less than 0 update count is entered, the item will be removed from cart.
Update the condition inside dc_ajax_add_cart_update_quantity_refresh():

  // If altered quantity is not zero, then update line item's quantity.
  // Otherwise delete line item.
  if ($form_state['values']['quantity_' . $form_state['triggering_element']['#line_item_id']] != 0) {
    ...
  }
  else {
    ...
  }

Remaining tasks

Make the changes and commit it.

User interface changes

N/A

API changes

N/A

Data model changes

N/A