Enable Lint/StringConversionInInterpolation rubocop rule

To prevent redundant `to_s` like https://github.com/rails/rails/pull/32923#discussion_r189460008
automatically in the future.

.rubocop.yml

@@ -162,6 +162,9 @@ Style/UnneededPercentQ:
 Lint/RequireParentheses:
   Enabled: true
 
+Lint/StringConversionInInterpolation:
+  Enabled: true
+
 Style/RedundantReturn:
   Enabled: true
   AllowMultipleReturnValues: true

activerecord/lib/active_record/log_subscriber.rb

@@ -104,7 +104,7 @@ module ActiveRecord
 
         if source_line
           if defined?(::Rails.root)
-            app_root = "#{::Rails.root.to_s}/".freeze
+            app_root = "#{::Rails.root}/"
             source_line = source_line.sub(app_root, "")
           end
 

activerecord/test/cases/arel/support/fake_record.rb

@@ -51,11 +51,11 @@ module FakeRecord
     end
 
     def quote_table_name(name)
-      "\"#{name.to_s}\""
+      "\"#{name}\""
     end
 
     def quote_column_name(name)
-      "\"#{name.to_s}\""
+      "\"#{name}\""
     end
 
     def schema_cache

railties/test/application/server_test.rb

@@ -36,7 +36,7 @@ module ApplicationTests
       skip "PTY unavailable" unless available_pty?
 
       File.open("#{app_path}/config/boot.rb", "w") do |f|
-        f.puts "ENV['BUNDLE_GEMFILE'] = '#{Bundler.default_gemfile.to_s}'"
+        f.puts "ENV['BUNDLE_GEMFILE'] = '#{Bundler.default_gemfile}'"
         f.puts "require 'bundler/setup'"
       end