Extract#as should not mutate the receiver

Fixes https://github.com/rails/rails/issues/16913

lib/arel/nodes/extract.rb

@@ -1,20 +1,14 @@
 module Arel
   module Nodes
     class Extract < Arel::Nodes::Unary
+      include Arel::AliasPredication
       include Arel::Predications
 
       attr_accessor :field
-      attr_accessor :alias
 
-      def initialize expr, field, aliaz = nil
+      def initialize expr, field
         super(expr)
         @field = field
-        @alias = aliaz && SqlLiteral.new(aliaz)
-      end
-
-      def as aliaz
-        self.alias = SqlLiteral.new(aliaz)
-        self
       end
 
       def hash
@@ -23,8 +17,7 @@ module Arel
 
       def eql? other
         super &&
-          self.field == other.field &&
-          self.alias == other.alias
+          self.field == other.field
       end
       alias :== :eql?
     end

lib/arel/visitors/to_sql.rb

@@ -475,14 +475,7 @@ module Arel
 
       def visit_Arel_Nodes_Extract o, collector
         collector << "EXTRACT(#{o.field.to_s.upcase} FROM "
-        collector = visit o.expr, collector
-        collector << ")"
-        if o.alias
-          collector << " AS "
-          visit o.alias, collector
-        else
-          collector
-        end
+        visit(o.expr, collector) << ")"
       end
 
       def visit_Arel_Nodes_Count o, collector

test/nodes/test_extract.rb

@@ -15,6 +15,14 @@ describe Arel::Nodes::Extract do
         EXTRACT(DATE FROM "users"."timestamp") AS foo
       }
     end
+
+    it 'should not mutate the extract' do
+      table = Arel::Table.new :users
+      extract = table[:timestamp].extract('date')
+      before = extract.dup
+      extract.as('foo')
+      assert_equal extract, before
+    end
   end
 
   describe 'equality' do