How to select tags with namespaces in CasperJS?

I'm refactoring a RSS so I decided to write some tests with CasperJS.

One of the elements of the RSS is "atom:link" (")

I tried this threecodes, but none one works

test.assertExists("atom:link", "atom:link tag exists.");

test..assertExists({
                type: 'xpath',
                path: "/atom:link"
            }, "atom:link element exists.");
//even this...
test.assertExists({
            type: 'xpath',
            namespace: "xmlns:atom",
            path: "atom:link"
        }, "atom:link element exists.");

Anyone know how to select this kind of elements with a namespace?

Thanks