styled-components example doesn't server render well

[sergiodxa]

I tried the styled-components example and in the server render process it writes the styles directly in the HTML instead of rendering them inside a <style> tag.

Example with JS disabled

Example with JS enabled

This is due this line (https://github.com/zeit/next.js/blob/master/examples/with-styled-components/pages/_document.js#L7) return an string without the <style> tag and Next.js expect the style prop to have the tag.