Fixed missing isset in constructor check

Author: acrobat

Diff for: src/CachePlugin.php

@@ -62,7 +62,7 @@ public function __construct(CacheItemPoolInterface $pool, StreamFactory $streamF
         $this->pool = $pool;
         $this->streamFactory = $streamFactory;
 
-        if (isset($config['respect_cache_headers']) && $config['respect_response_cache_directives']) {
+        if (isset($config['respect_cache_headers']) && isset($config['respect_response_cache_directives'])) {
             throw new \InvalidArgumentException(
                 'You can\'t provide config option "respect_cache_headers" and "respect_response_cache_directives". '.
                 'Use "respect_response_cache_directives" instead.'