BUGFIX: Fix ActionResponse to handle BufferStream

```
Cannot seek a BufferStream

GuzzleHttp\Psr7\BufferStream::seek(0)
GuzzleHttp\Psr7\BufferStream::rewind()
Neos\Flow\Mvc\ActionResponse::getContent()
Neos\FluidAdaptor\Core\Widget\AbstractWidgetViewHelper::initiateSubRequest(
```

Neos.Flow/Classes/Mvc/ActionResponse.php

@@ -198,7 +198,9 @@ public function getHttpHeader(string $headerName)
     public function getContent(): string
     {
         $content = $this->content->getContents();
-        $this->content->rewind();
+        if ($this->content->isSeekable()) {
+            $this->content->rewind();
+        }
         return $content;
     }