Issue #1056130 by ogi: PHP notice that ->cache is undefined.
Issue #1144062 by mr.baileys: Changes to hook_requirements.
Issue #1015886 by lmichot: Function drupal_get_headers() does not exist in D7
Issue #1194056 by catch: apc_fetch() accepts an array - use for ->getMultiple()?
* apc_store is already serializing because Drupal saves an object. Simple data structures do not need to be serialized twice #951838.
* Some small changes to be able to run the Drupal Cache Tests while using APC.