Javascript – ajax, setRequestHeader(), Content-Type, application/x-www-form-urlencoded and charset

I am having trouble understanding how to set the charset when the
content type is not text/html, text/plain, or text/xml, but is application/x-www-form-urlencoded content type instead.

Given this (simplified) javascript code:

var xhr = new XMLHttpRequest();

If I do not explicitly set the encoding,

xhr.open('POST', 'serv.php', true);
xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');

firebug tells me that the content
type is "application/x-www-form-urlencoded; charset=UTF-8."

If I set the charset to ISO-8859-1 for instance,

xhr.open('POST', 'serv.php', true);
xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=ISO-8859-1');

firebug still tells me "application/x-www-form-urlencoded; charset=UTF-8."

If I try something like

xhr.setRequestHeader('Content-Type', 'text/plain; charset=ISO-8859-1');

then it respects the charset.

In all the cases the send() method goes like this:

xhr.send('id=9&name=Yoda');

Why doesn't it honor the charset I specify if the Content-Type is x-www-form-urlencoded?

NOTE: I am using ISO-8859-1 just as an example. My goal is to understand what is going on.