* Fix: content.php: last-modified/if-unmodified-since (#3771)
Fix exception if server sent invalid Last-Modified header
Add support for Unix time instead of standard date string
Send back standard RFC7231 date string instead of Unix time
* Fix: content.php: if-unmodified-since: cURL API
Use getTimestamp() as cURL expects that and will format the If-Modified-Since header appropriately.