Return directive type for method Stream::getMetadata() is not valid

https://github.com/mpdf/psr-http-message-shim/blame/f25a0153d645e234f9db42e5433b16d9b113920f/src/Stream.php#L256

Return type for the method getMetadata() is "bool", but the returned data in the method differs greatly: I'd like to suggest to remove the ": bool" directive. 