Freeze all the strings in visitors

lib/arel/visitors/bind_visitor.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     module BindVisitor

lib/arel/visitors/dot.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class Dot < Arel::Visitors::Visitor
@@ -272,7 +273,7 @@ module Arel
             label = "<f0>#{node.name}"
 
             node.fields.each_with_index do |field, i|
-              label << "|<f#{i + 1}>#{quote field}"
+              label += "|<f#{i + 1}>#{quote field}"
             end
 
             "#{node.id} [label=\"#{label}\"];"

lib/arel/visitors/ibm_db.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class IBM_DB < Arel::Visitors::ToSql

lib/arel/visitors/informix.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class Informix < Arel::Visitors::ToSql

lib/arel/visitors/mssql.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class MSSQL < Arel::Visitors::ToSql

lib/arel/visitors/mysql.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class MySQL < Arel::Visitors::ToSql

lib/arel/visitors/oracle.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class Oracle < Arel::Visitors::ToSql
@@ -125,7 +126,7 @@ module Arel
             array[i] << ',' << part
           else
             # to ensure that array[i] will be String and not Arel::Nodes::SqlLiteral
-            array[i] = '' << part
+            array[i] = part.to_s
           end
           i += 1 if array[i].count('(') == array[i].count(')')
         end

lib/arel/visitors/oracle12.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class Oracle12 < Arel::Visitors::ToSql

lib/arel/visitors/postgresql.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class PostgreSQL < Arel::Visitors::ToSql

lib/arel/visitors/sqlite.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class SQLite < Arel::Visitors::ToSql

lib/arel/visitors/to_sql.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'bigdecimal'
 require 'date'
 require 'arel/visitors/reduce'

lib/arel/visitors/where_sql.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class WhereSql < Arel::Visitors::ToSql