Fixed compatibility issue with Symfony 5 (#21)

Author: Toflar

src/Psr6Store.php

@@ -254,12 +254,10 @@ public function write(Request $request, Response $response)
         // Tags
         $tags = [];
         if ($response->headers->has($this->options['cache_tags_header'])) {
-            // Symfony < 4.4
-            $headers = $response->headers->get($this->options['cache_tags_header'], '', false);
-            if (\is_string($headers)) {
-                // Symfony >= 4.4
-                $headers = $response->headers->all($this->options['cache_tags_header']);
-            }
+            // Compatibility with Symfony 3+
+            $allHeaders = $response->headers->all();
+            $key = str_replace('_', '-', strtolower($this->options['cache_tags_header']));
+            $headers = isset($allHeaders[$key]) ? $allHeaders[$key] : [];
 
             foreach ($headers as $header) {
                 foreach (explode(',', $header) as $tag) {