Rails 3.2 - No RoutingError

I'm currently updating a Rails Application from 3.1 to 3.2.
While trying to fix a failing test I noticed, that when you try to
generate an Url for a non existant Controller, this will not raise a
RoutingError, but generate an assets Url.

I also tried it in the rails console of a fresh generated app (3.2.1):

1.9.3p0 :001 > app.url_for :controller => 'foo'
=> "http://www.example.com/assets?controller=foo"
1.9.3p0 :002 >

The Problem is, that there are some functional tests in our
application which rely on pre 3.1 behaviour of raising RoutingErrors
when no route matches the provided params.

In my opinion, trying to generate a url where the params doesn't match
any route should clearly raise an exception.