FIX: couldn't uncategorize a topic

app/controllers/topics_controller.rb

@@ -133,7 +133,7 @@ class TopicsController < ApplicationController
     end
 
     changes.delete(:title) if topic.title == changes[:title]
-    changes.delete(:category_id) if (changes[:category_id].blank? or topic.category_id == changes[:category_id].to_i)
+    changes.delete(:category_id) if topic.category_id == changes[:category_id].to_i
 
     success = true
     if changes.length > 0

spec/controllers/topics_controller_spec.rb

@@ -724,6 +724,11 @@ describe TopicsController do
           xhr :put, :update, topic_id: @topic.id, slug: @topic.title, category_id: 123
         end
 
+        it 'allows to change category to "uncategorized"' do
+          Topic.any_instance.expects(:change_category_to_id).with(0).returns(true)
+          xhr :put, :update, topic_id: @topic.id, slug: @topic.title, category_id: ""
+        end
+
         it "returns errors with invalid titles" do
           xhr :put, :update, topic_id: @topic.id, slug: @topic.title, title: 'asdf'
           expect(response).not_to be_success