Empty body for response with psr/http-message v2

[Axent96]

"psr/http-message": "^2.0"

The src/Helper.php need to be updated
` if ($httpMessage instanceof ResponseInterface) {
$xmlString = (string)$httpMessage->getBody();

        $xmlString = simplexml_load_string($xmlString);
        return static::parseXmlElement($xmlString);
    }

to
if ($httpMessage instanceof ResponseInterface) {
$xmlString = $httpMessage->getBody()->getContents();

        $xmlString = simplexml_load_string($xmlString);
        return static::parseXmlElement($xmlString);
    }

`

[Axent96]

#17 Pull Request