Support weak (W/) Etags/If-None-Match header

[cdrini]

Etags/If-None-Match can be optionally prefixed by W/ to denote a "weak validator" according to MDN. This flag denotes that it is approximately identical, but not byte-identical. MDN suggests W/ shouldn't match for byte ranges. Presumably it should match for anything else though?

Update http.modified to handle W/.

MDN states:

If-None-Match
Succeeds if the ETag of the distant resource is different to each listed in this header. By default, unless the etag is prefixed with 'W/', it performs a strong validation.

So if the requester etag is weak, then if the page etag can be weak or strong. If it's strong, then the page's etag must also be strong. I believe that's the correct interpretation.