Must support aliases for OVER operator.

lib/arel/nodes/over.rb

@@ -2,6 +2,8 @@ module Arel
   module Nodes
 
     class Over < Binary
+      include Arel::AliasPredication
+
       def initialize(left, right = nil)
         super(left, right)
       end

test/nodes/test_over.rb

@@ -1,6 +1,15 @@
 require 'helper'
 
 describe Arel::Nodes::Over do
+  describe 'as' do
+    it 'should alias the expression' do
+      table = Arel::Table.new :users
+      table[:id].count.over.as('foo').to_sql.must_be_like %{
+        COUNT("users"."id") OVER () AS foo
+      }
+    end
+  end
+
   describe 'with literal' do
     it 'should reference the window definition by name' do
       table = Arel::Table.new :users