HTTP: If-Modified-Since now allow clients from the future.

Before this patch the server did not respect a conditional
If-Modified-Since if the value was higher than the system server
date. This patch makes this pass, not a bug but make it more
friendly with Checker tools.

This patch also fix a QA script involved.

Signed-off-by: Eduardo Silva <eduardo@monkey.io>
HTTP: on using API sendfile, check EAGAIN

Some plugins may not be too efficient to buffer outgoing data
and when implementing the sendfile callback on some cases it will
need to re-call the routine until the Kernel enqueue the data.

Signed-off-by: Eduardo Silva <eduardo@monkey.io>
Version: bump to 1.5.2

Signed-off-by: Eduardo Silva <eduardo@monkey.io>
Update ChangeLog for v1.5.2

Signed-off-by: Eduardo Silva <eduardo@monkey.io>