Fixed missing visibility and comments of on internal methods

I'm aware that this is technically a BC break but those methods were never supposed to be called as there's absolutely no reason to do so.
Calling those should be considered a bug.
I've also double checked in all Composer dependents (dependent packages) and couldn't find anyone using those.

Author: Toflar

src/Psr6Store.php

@@ -399,6 +399,8 @@ public function clear()
 
     /**
      * @return string
+     *
+     * @internal Do not use in public code, this is for unit testing purposes only
      */
     public function getCacheKey(Request $request)
     {
@@ -409,10 +411,24 @@ public function getCacheKey(Request $request)
         return 'md'.hash('sha256', $uri);
     }
 
+    /**
+     * @return string
+     *
+     * @internal Do not use in public code, this is for unit testing purposes only
+     */
+    public function generateContentDigest(Response $response)
+    {
+        if ($response instanceof BinaryFileResponse) {
+            return 'bf'.hash_file('sha256', $response->getFile()->getPathname());
+        }
+
+        return 'en'.hash('sha256', $response->getContent());
+    }
+
     /**
      * @return string
      */
-    public function getVaryKey(array $vary, Request $request)
+    private function getVaryKey(array $vary, Request $request)
     {
         if (0 === \count($vary)) {
             return self::NON_VARYING_KEY;
@@ -442,18 +458,6 @@ public function getVaryKey(array $vary, Request $request)
         return hash('sha256', $hashData);
     }
 
-    /**
-     * @return string
-     */
-    public function generateContentDigest(Response $response)
-    {
-        if ($response instanceof BinaryFileResponse) {
-            return 'bf'.hash_file('sha256', $response->getFile()->getPathname());
-        }
-
-        return 'en'.hash('sha256', $response->getContent());
-    }
-
     private function saveContentDigest(Response $response)
     {
         if ($response->headers->has('X-Content-Digest')) {