[HttpClient] Fix buffering after calling AsyncContext::passthru()

Author: nicolas-grekas

Response/AsyncContext.php

@@ -181,9 +181,15 @@ public function replaceResponse(ResponseInterface $response): ResponseInterface
 
     /**
      * Replaces or removes the chunk filter iterator.
+     *
+     * @param ?callable(ChunkInterface, self): ?\Iterator $passthru
      */
     public function passthru(callable $passthru = null): void
     {
-        $this->passthru = $passthru;
+        $this->passthru = $passthru ?? static function ($chunk, $context) {
+            $context->passthru = null;
+
+            yield $chunk;
+        };
     }
 }