Clarify sniffing of empty resources

[annevk]

From https://www.w3.org/Bugs/Public/show_bug.cgi?id=25190 by Robin Berjon:

It is unclear to me what empty payloads with no labelling content-type are supposed to be sniffed as.

Following the algorithm, I eventually reach:

"""
9. If the resource header contains no binary data bytes, the sniffed MIME type is "text/plain". > Abort these steps.
10. The sniffed MIME type is "application/octet-stream".
"""

It's not entirely clear that if the resource header contains nothing then it can be said to contain no binary data bytes. (Or rather, at the very least the case seems to be treated differently by implementations.)

In testing of such a resource (loaded into an iframe), Gecko prompts for download whereas Blink, WebKit, and Trident all report the document.contentType as undefined.

It would be worth deciding what empty unlabelled payloads should sniff as.

Seems to me this should be text/plain, but would be good to verify with tests.