[XHR] Correction in getResponseHeader behaviour

Hi,

while trying to correct getResponseHeader behaviour on WebKit (
http://bugs.webkit.org/show_bug.cgi?id=15356), it appears that most of the
browsers do raise a SYNTAX_ERR exception when the header argument does not
match the field-name production (point 3 in getResponseHeader description).

The behaviour is to return an empty string on IE/Opera and return null on
Firefox.

WebKit has sided with the first option so we suggest that the draft is
updated to return an empty string in that case.

Regards,

Julien