private String getPayload(HttpServletRequest req) {
        ServletInputStream is = null;
        try {
            is = req.getInputStream();
            return IOUtils.toString(is);
        } catch (IOException e) {
            throw new RuntimeException(e);
        } finally {
            IOUtils.closeQuietly(is);
        }
    }
Gets the payload body and returns it as a string.
@param req
manager/test/api/src/main/java/io/apiman/manager/test/server/MockGatewayServlet.java