Strange problem on "xhr" and "xml_http_request"

Hi,

I'm learning the integration test framework of rails, it's definitely
great! But when I tried to use xml_http_request's alias xhr, test
broken ...

If I wrote

xml_http_request :post, "/store/add_to_cart", :id => ruby_book.id

everything's ok, tests passed succesfully. After I change this line
to

xhr :post, "/store/add_to_cart", :id => ruby_book.id

I get exceptions like this:

Loaded suite test/integration/user_stories_test
Started
E
Finished in 1.203 seconds.

  1) Error:
test_buying_a_product(UserStoriesTest):
ArgumentError: wrong number of arguments (4 for 3)
    /usr/lib/ruby/gems/1.8/gems/actionpack-1.13.5/lib/
action_controller/integration.rb:508:in `post'
    /usr/lib/ruby/gems/1.8/gems/actionpack-1.13.5/lib/
action_controller/integration.rb:508:in `send'
    /usr/lib/ruby/gems/1.8/gems/actionpack-1.13.5/lib/
action_controller/integration.rb:508:in `post'
    /usr/lib/ruby/gems/1.8/gems/actionpack-1.13.5/lib/
action_controller/test_process.rb:391:in `send'
    /usr/lib/ruby/gems/1.8/gems/actionpack-1.13.5/lib/
action_controller/test_process.rb:391:in `xhr'
    test/integration/user_stories_test.rb:22:in
`test_buying_a_product'
    /usr/lib/ruby/gems/1.8/gems/actionpack-1.13.5/lib/
action_controller/integration.rb:463:in `run'

1 tests, 2 assertions, 0 failures, 1 errors

I don't know why, xhr is just an alias of xml_http_request in the
source, isn't?

Thanks,
Jan