FIX: Allow duplicate slugs when the parent category is not the same

app/models/category.rb

@@ -187,7 +187,7 @@ SQL
 
       # If a category with that slug already exists, set the slug to nil so the category can be found
       # another way.
-      category = Category.where(slug: self.slug)
+      category = Category.where(slug: self.slug, parent_category_id: parent_category_id)
       category = category.where("id != ?", id) if id.present?
       self.slug = '' if category.exists?
     end

spec/models/category_spec.rb

@@ -243,11 +243,18 @@ describe Category do
     end
 
     describe "creating a new category with the same slug" do
-      it "should have a blank slug" do
+      it "should have a blank slug if at the same level" do
         category = Fabricate(:category, name: "Amazing Categóry")
         category.slug.should be_blank
         category.slug_for_url.should == "#{category.id}-category"
       end
+
+      it "doesn't have a blank slug if not at the same level" do
+        parent = Fabricate(:category, name: 'Other parent')
+        category = Fabricate(:category, name: "Amazing Categóry", parent_category_id: parent.id)
+        category.slug.should == 'amazing-category'
+        category.slug_for_url.should == "amazing-category"
+      end
     end
 
     describe "trying to change the category topic's category" do