Fix toHTML() for namespaced attributes

Without this fix, `ractive.toHTML()` strips the namespaces from attributes in the template, which is problematic for using things like the `xlink:href` attribute for svg:

    <svg viewBox='0 0 10 10'><use xlink:href='/vector.svg#check'></use></svg>

To fix this, I've changed the attribute/toString() method to preserve the namespace.

I added a test to `test/samples/render.js`, which passes when running `grunt nodeunit`, but fails when running `grunt qunit`. I think this has to do with PhantomJS's implementation of `innerHTML` when using `setAttributeNS`. I'm not sure how to proceed. Any guidance to get the suite passing here would be helpful.

Thanks!