Issue title: ElggMemcache->load reports false negative (ERROR: MEMCACHE: FAILED TO LOAD ...) (Trac #3993)
 Issue body: _Original ticket http://trac.elgg.org/ticket/3993 on 41805790-08-18 by trac user janlb, assigned to unknown._

Elgg version: 1.7

ElggMemcache->load($key) reports a false negative when it returns a value that evaluates to FALSE.

For example:

$memcache->save('key', array());
$memcache->load('key');

Will report that it failed to load 'key'

Elgg should check for FALSE 'identicalness' via ===
(see: http://www.php.net/manual/en/memcache.get.php#refsect1-memcache.get-returnvalues)

 Comments: 
 Comment 0: _trac user janlb wrote on 41825237-10-02_

duplicate of #1995

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