Fix bug: Headers string not appending \r
Created by: piersoh
The FetchLoader() currently only adds '\n' which results in a concatenated headers string that is not the same as that produced by XMLHttpRequest() and thus not correctly parsed by the DashMetrics.parseResponseHeaders() function (https://github.com/Dash-Industry-Forum/dash.js/blob/development/src/dash/DashMetrics.js#L434)