Writer : add a space before "/>" of empty emements : as W3C guideline #846
Description
In the W3C guideline (C.2): https://www.w3.org/TR/xhtml1/#guidelines
Empty elements should be ended with " />" instead of "/>"
A space should be added at line:
https://github.com/tafia/quick-xml/blob/master/src/writer/async_tokio.rs#L28