Zend Data Cache returns null when record doesn't exist (reverted from commit…

framework/yii/caching/ZendDataCache.php

@@ -21,25 +21,6 @@ namespace yii\caching;
 class ZendDataCache extends Cache
 {
 	/**
-	 * Checks whether a specified key exists in the cache.
-	 * This can be faster than getting the value from the cache if the data is big.
-	 * In case a cache does not support this feature natively, this method will try to simulate it
-	 * but has no performance improvement over getting it.
-	 * Note that this method does not check whether the dependency associated
-	 * with the cached data, if there is any, has changed. So a call to [[get]]
-	 * may return false while exists returns true.
-	 * @param mixed $key a key identifying the cached value. This can be a simple string or
-	 * a complex data structure consisting of factors representing the key.
-	 * @return boolean true if a value exists in cache, false if the value is not in the cache or expired.
-	 */
-	public function exists($key)
-	{
-		$key = $this->buildKey($key);
-		$value = $this->getValue($key);
-		return $value !== null;
-	}
-
-	/**
 	 * Retrieves a value from cache with a specified key.
 	 * This is the implementation of the method declared in the parent class.
 	 * @param string $key a unique key identifying the cached value