This is what the draft spec says, what MSIE does, and what Firefox will do, according to <http://lists.w3.org/Archives/Public/public-webapi/2006Nov/0055.html>. Currently, WebKit always encodes sent documents as UTF-8, and doesn't serialize the XML declaration, while Firefox also uses UTF-8, but sends an XML declaration with a potentially mismatched encoding.
The specification enforces UTF-8 these days.