fix phpstan level 2

Author: dbu

src/Cache/Generator/HeaderCacheKeyGenerator.php

@@ -14,12 +14,12 @@ class HeaderCacheKeyGenerator implements CacheKeyGenerator
     /**
      * The header names we should take into account when creating the cache key.
      *
-     * @var array
+     * @var string[]
      */
     private $headerNames;
 
     /**
-     * @param $headerNames
+     * @param string[] $headerNames
      */
     public function __construct(array $headerNames)
     {

src/CachePlugin.php

@@ -333,7 +333,7 @@ private function getMaxAge(ResponseInterface $response)
         if (!is_bool($maxAge)) {
             $ageHeaders = $response->getHeader('Age');
             foreach ($ageHeaders as $age) {
-                return $maxAge - ((int) $age);
+                return ((int) $maxAge) - ((int) $age);
             }
 
             return (int) $maxAge;
@@ -449,14 +449,16 @@ private function getETag(CacheItemInterface $cacheItem)
         $data = $cacheItem->get();
         // The isset() is to be removed in 2.0.
         if (!isset($data['etag'])) {
-            return;
+            return null;
         }
 
         foreach ($data['etag'] as $etag) {
             if (!empty($etag)) {
                 return $etag;
             }
         }
+
+        return null;
     }
 
     /**