FIX: Improve Topic.similar_to with better Topic#title matches.

This changes PG text search to only match the given title against
lexemes that are formed from the title. Likewise, the given raw will
only be matched against lexemes that are formed from the post's raw.

spec/models/topic_spec.rb

@@ -502,37 +502,46 @@ describe Topic do
     end
   end
 
-  context 'similar_to' do
+  context '.similar_to' do
+    fab!(:category) { Fabricate(:category_with_definition) }
 
-    it 'returns blank with nil params' do
-      expect(Topic.similar_to(nil, nil)).to be_blank
+    it 'returns an empty array with nil params' do
+      expect(Topic.similar_to(nil, nil)).to eq([])
     end
 
     context "with a category definition" do
-      let!(:category) { Fabricate(:category_with_definition) }
-
       it "excludes the category definition topic from similar_to" do
-        expect(Topic.similar_to('category definition for', "no body")).to be_blank
+        expect(Topic.similar_to('category definition for', "no body")).to eq([])
       end
     end
 
     context 'with a similar topic' do
-      let!(:topic) {
+      fab!(:post) {
         SearchIndexer.enable
-        post = create_post(title: "Evil trout is the dude who posted this topic")
-        post.topic
+        create_post(title: "Evil trout is the dude who posted this topic")
       }
 
+      let(:topic) { post.topic }
+
+      before do
+        SearchIndexer.enable
+      end
+
       it 'returns the similar topic if the title is similar' do
         expect(Topic.similar_to("has evil trout made any topics?", "i am wondering has evil trout made any topics?")).to eq([topic])
       end
 
-      context "secure categories" do
-        fab!(:category) { Fabricate(:category_with_definition, read_restricted: true) }
+      it 'matches title against title and raw against raw when searching for topics' do
+        topic.update!(title: '1 2 3 numbered titles')
+        post.update!(raw: 'random toy poodle')
 
+        expect(Topic.similar_to("unrelated term", "1 2 3 poddle")).to eq([])
+      end
+
+      context "secure categories" do
         before do
-          topic.category = category
-          topic.save
+          category.update!(read_restricted: true)
+          topic.update!(category: category)
         end
 
         it "doesn't return topics from private categories" do