use eager instead of lazy cache for device detector, as cache should be more...

use eager instead of lazy cache for device detector, as cache should be more persistant as it can be used in each tracking request

core/DeviceDetectorCache.php

@@ -28,7 +28,7 @@ class DeviceDetectorCache implements \DeviceDetector\Cache\Cache
     public function __construct($ttl = 300)
     {
         $this->ttl   = (int) $ttl;
-        $this->cache = PiwikCache::getLazyCache();
+        $this->cache = PiwikCache::getEagerCache();
     }
 
     /**
@@ -47,6 +47,10 @@ class DeviceDetectorCache implements \DeviceDetector\Cache\Cache
             return self::$staticCache[$id];
         }
 
+        if (!$this->cache->contains($id)) {
+            return false;
+        }
+
         return $this->cache->fetch($id);
     }