Fixes #453: Zend_Cache_Backend_Memcached looks at "bytes", but Couchbase 1.x returns "mem_used"

Closes #463

library/Zend/Cache/Backend/Memcached.php

@@ -390,7 +390,12 @@ class Zend_Cache_Backend_Memcached extends Zend_Cache_Backend implements Zend_Ca
             }
 
             $eachSize = $mem['limit_maxbytes'];
-            $eachUsed = $mem['bytes'];
+
+            /**
+             * Couchbase 1.x uses 'mem_used' instead of 'bytes'
+             * @see https://www.couchbase.com/issues/browse/MB-3466
+             */
+            $eachUsed = isset($mem['bytes']) ? $mem['bytes'] : $mem['mem_used'];
             if ($eachUsed > $eachSize) {
                 $eachUsed = $eachSize;
             }