Validator should send HTTP_ACCEPT application/xhtml+xml header

My code detects whether browser can accept xhtml+xml or not based upon 
whether the client tells the server it can.

Browsers that accept xhtml+xml get xhtml version of page, browsers that 
do not get html version of page.

Interestingly, when I validate my pages by URL, your validator validates 
the html version of the page, indicating that your validator doesn't 
tell my server it accepts xhtml+xml

This is a minor issue, but since your validator accepts xhtml+xml it 
probably should tell the server it requests a page from that it does 
because some pages (like mine) do send different output based upon what 
the requesting client says it can handle.