[whatwg] several messages about content sniffing in HTML

[Boris Zbarsky]

Geoffrey Sneddon wrote:
> Content-Type: XML
> Content-Type: text/XML
> 
> Using the first would break badly. I guess it seems to work because of 
> content-type sniffing on an unknown (and invalid) header

Or because the header parser uses the first header that actually looks like a 
valid content-type (e.g. contains a '/').  Specifying this _is_ needed.

-Boris