Add mixed return type

Fixes the following error caught by Symfony's PHPUnit Bridge:

```
  1x: Method "Psr\Http\Message\StreamInterface::getMetadata()" might add "mixed" as a native return type declaration in the future. Do the same in implementation "Http\Message\Encoding\FilteredStream" now to avoid errors or add an explicit @return annotation to suppress this message.
```

src/Decorator/StreamDecorator.php

@@ -86,6 +86,9 @@ public function getContents(): string
         return $this->stream->getContents();
     }
 
+    /**
+     * @return mixed
+     */
     public function getMetadata(string $key = null)
     {
         return $this->stream->getMetadata($key);