Fix bug discovered in r77171 from the user data in If-Modified-Since passed by resour...

Fix bug discovered in r77171 from the user data in If-Modified-Since passed by resource loader.
Added in wfTimestamp reading support for the three http date formats.
Increased conformance reading rfc2822 dates (read support added in r71750/r71751). We may not want full compliance with rfc2822, though.
The only provider of rfc2822 dates is probably http and that uses a subset (it's rfc822 + 4-digit years from rfc1123).
Make wfTimestamp() return false in case of a wrong input, according to CR.