Describe strings within data_driven hook not updated

[4ndrewCox]

The keywords within the describe() string are not replaced with the test data {width} and {height}

data_driven(viewportSizes, function () {

            describe('Browser Viewport for width {width} and height {height}', function () {

                before(function (ctx) {
                    browser.setViewportSize({width: ctx.width, height: ctx.height});
                });