Writer : add a space before "/>" of empty emements : as W3C guideline

[rubik83]

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

[Mingun]

I'm open for a PR which will add an option to Writer to do that. It should be documented and tested (doctest will be enough). Because currently Writer does not have options struct like a Reader, the preferred way is to add it.