minitest ignores assert_response() message

Hello,

I'm using Rails 3.0.6 and I find that minitest (both the Ruby 1.9.2-p180
version and also the minitest 2.0.2 gem) is ignoring the message I'm
passing to assert_response():

      post :create, content: File.read(file)
      assert_response :success, response.body

When the assertion fails, minitest is not showing my `response.body`
message and is instead always showing "Expected block to return true
value." as the message.

Thanks for your consideration.

P.S. Test::Unit 2.2.0 does not exhibit this problem.