How to use http source for pulling JS in jasmine-rails gem?

In application layout loading the http source occurs as follows:

= javascript_include_tag "//example.source.com/external.js"

But when I starting the tests RAILS_ENV=test rake spec:javascript I get the following error: ERROR: ReferenceError: Can't find variable: externalVar.

How to correctly use http resource for pulling JS in jasmine-rails gem?