"elem.attributes" should include "style" attribute

Currently, `elem.attributes` does not include the `style` attribute since that's stored as a separate object. See: https://github.com/developit/undom/blob/master/src/undom.js#L93-L95

This differs from my expectations. It also means that the provided `serializeHtml()` function doesn't include inline styles without modification.

Update: also, `elem.style.cssText` returns an object when a string is expected. Is this level of DOM compliance out-of-scope for `undom` or is this a reasonable expectation?