AJAX Explicitly Set The Encoding When Sending Out The Post Request

I am trying to send request using ajax with some japanese text.
As any of you know XMLHttpRequest is send with utf-8.

I was hoping there is a way to send with different encoding like

var parameters = "metadata=" +
encodeURI(document.getElementById("metadata").value);

http_request.onreadystatechange = metadataUpdate;
http_request.open( 'POST', "/servlet/SendJapText", true );
http_request.setRequestHeader( "Content-type",
"application/x-www-form-urlencoded;charset=Shift-JIS" );
http_request.send( parameters );