XMLHttpRequest.getResponseHeader and missing headers

"If no headers of that name were received, then it MUST return the
empty string."

  This matches the behavior of Internet Explorer and Opera 9, but not
of Firefox 1.5 or Safari 2 (the former returns null, while the latter
returns undefined).

  I don't see any restriction on the semantics of empty vs. missing
headers in RFC 2616, and indeed, the difference is sometimes
significant, at least in the case of requests (SOAP, Atom API). Thus,
the Firefox behavior appears the most reasonable to me. It is
documented in the idl at
<http://lxr.mozilla.org/mozilla/source/content/base/public/nsIXMLHttpRequest.idl#148>.

  Can the specification be adjusted to match it at this point?

- WBR, Alexey Proskuryakov