Bug 4471: revalidation doesn't work when expired cached object lacks Last-Modified.

Bug 4471: revalidation doesn't work when expired cached object lacks Last-Modified.

The bug was caused by the fact that Squid used only Last-Modified header
value for evaluating entry's last modification time while making an
internal revalidation request. So, without Last-Modified it was not
possible to correctly fill If-Modified-Since header value. As a result,
the revalidation request was not initiated when it should be.

To fix this problem, we should generate If-Modified-Since based on other
data, showing how "old" the cached response is, such as Date and Age
header values. Both Date and Age header values are utilized while
calculating entry's timestamp. Therefore, we should use the timestamp if
Last-Modified is unavailable.

TODO: HttpRequest::imslen support looks broken/deprecated. Using this
field inside StoreEntry::modifiedSince() leads to several useless checks
and probably may cause other [hidden] problems.