Add Nodes::TableAlias#engine

Eventually #engine should go away, but until that time, this means that
Table and Nodes::TableAlias can be used more interchangeably.

lib/arel/nodes/table_alias.rb

@@ -12,6 +12,10 @@ module Arel
       def table_name
         relation.respond_to?(:name) ? relation.name : name
       end
+
+      def engine
+        relation.engine
+      end
     end
   end
 end

test/nodes/test_table_alias.rb

@@ -0,0 +1,16 @@
+require 'helper'
+require 'ostruct'
+
+module Arel
+  module Nodes
+    describe 'table alias' do
+      it 'has an #engine which delegates to the relation' do
+        engine   = Object.new
+        relation = OpenStruct.new(:engine => engine)
+
+        node = TableAlias.new relation, :foo
+        node.engine.must_equal engine
+      end
+    end
+  end
+end