Fix modifying frozen strings errors take 3.

app/controllers/session_controller.rb

@@ -28,7 +28,7 @@ class SessionController < ApplicationController
 
     if destination_url && return_path == path('/')
       uri = URI::parse(destination_url)
-      return_path = "#{uri.path}#{uri.query ? "?" << uri.query : ""}"
+      return_path = "#{uri.path}#{uri.query ? "?#{uri.query}" : ""}"
     end
 
     session.delete(:destination_url)
@@ -229,7 +229,7 @@ class SessionController < ApplicationController
       render_sso_error(text: text || I18n.t("sso.unknown_error"), status: 500)
 
     rescue => e
-      message = "Failed to create or lookup user: #{e}."
+      message = +"Failed to create or lookup user: #{e}."
       message << "  "
       message << "  #{sso.diagnostics}"
       message << "  "

app/models/topic.rb

@@ -1001,7 +1001,7 @@ class Topic < ActiveRecord::Base
   end
 
   def self.url(id, slug, post_number = nil)
-    url = "#{Discourse.base_url}/t/#{slug}/#{id}"
+    url = +"#{Discourse.base_url}/t/#{slug}/#{id}/"
     url << "/#{post_number}" if post_number.to_i > 1
     url
   end

app/models/topic_tracking_state.rb

@@ -168,7 +168,7 @@ class TopicTrackingState
     #
     # This code needs to be VERY efficient as it is triggered via the message bus and may steal
     #  cycles from usual requests
-    sql = report_raw_sql(
+    sql = +report_raw_sql(
       topic_id: topic_id,
       skip_unread: true,
       skip_order: true,