Optimize ActiveRecord::Result#last

If you only want the last element of a result set, there's no need to create all
of hash_rows. Also, add a test.

activerecord/lib/active_record/result.rb

@@ -81,7 +81,8 @@ def first
     end
 
     def last
-      hash_rows.last
+      return nil if @rows.empty?
+      Hash[@columns.zip(@rows.last)]
     end
 
     def cast_values(type_overrides = {}) # :nodoc:

activerecord/test/cases/result_test.rb

@@ -27,6 +27,11 @@ def result
         {'col_1' => 'row 1 col 1', 'col_2' => 'row 1 col 2'}, result.first)
     end
 
+    test "last returns last row as a hash" do
+      assert_equal(
+        {'col_1' => 'row 3 col 1', 'col_2' => 'row 3 col 2'}, result.last)
+    end
+
     test "each with block returns row hashes" do
       result.each do |row|
         assert_equal ['col_1', 'col_2'], row.keys