integration test redirects to external URLs

In an integration test that I am writing I want to do something like

def test_my_page
    get "/rails/someaction"
     assert_response :redirect
     follow_redirect!
     assert_url => 'http://www.somepageoutsidemyrailsapp'
end

Is there any way to do something like 'assert_url' and see if I was redirected to a page outside of the rails application? If the page was part of my rails app I could use assert_redirected_to but since I want to check against a full URL I don't know how to proceed.

Thank you,
Matt Margolis