Handle negative numbers in Redis correctly.

Update number sniff to handle negative numbers. We need to do number
sniffing so we can maintain compatbility between write() and
increment()/decrement().

Refs #8364

lib/Cake/Cache/Engine/RedisEngine.php

@@ -133,11 +133,11 @@ class RedisEngine extends CacheEngine {
  */
 	public function read($key) {
 		$value = $this->_Redis->get($key);
-		if (ctype_digit($value)) {
-			$value = (int)$value;
+		if (preg_match('/^[-]?\d+$/', $value)) {
+			return (int)$value;
 		}
 		if ($value !== false && is_string($value)) {
-			$value = unserialize($value);
+			return unserialize($value);
 		}
 		return $value;
 	}