[Issue #24] Display haggling modal

app/controllers/sales_controller.rb

@@ -325,7 +325,7 @@ class SalesController < ApplicationController
 
   def add_haggle
     @pre_sale = PreSale.find(params[:pre_sale])
-    @haggle_percent = current_user.pointsale.haggle_percent.zero? ? @pos_config.haggle_in_sale_percent : current_user.pointsale.haggle_percent
+    @haggle_percent = !current_user.pointsale.haggle_percent.blank? ? current_user.pointsale.haggle_percent : @pos_config.haggle_in_sale_percent
     @suggested_haggle = (@haggle_percent.to_f / 100) * @pre_sale.unit_price
   end