config.cache_classes and ActionMailer::Base.default_url_options

I have this is my test.rb:

config.cache_classes = true
config.action_mailer.default_url_options = {
  :host => 'localhost:3000',
  :only_path => false
}

When running tests, I get:

/Users/sebastian/.rvm/gems/ruby-1.9.2-p136/gems/actionmailer-3.0.3/lib/action_mailer/base.rb:763:in deprecated_url_options': You can no longer call ActionMailer::Base.default_url_options directly. You need to set config.action_mailer.default_url_options. If you are using ActionMailer standalone, you need to include the routing url_helpers directly.

This problem does not occur, when using:

config.cache_classes = false