Secure product_categories and add example test for other supplier resources and fix hack attempts included in test

app/models/product_category.rb

@@ -12,6 +12,7 @@ class ProductCategory
   validates :name, presence: true
   validates :position, numericality: true
   validates :supplier_id, presence: true
+  view :by_supplier_id_and_id, key: [:supplier_id, :_id]
 
   def self.for_user(user, options = {})
     table = options[:table]