HTTPStatusCode 301 - Moved Support

[moflaherty]

Hi! Possible suggestion for supporting both 301 and 308 status codes? Reference: https://stackoverflow.com/questions/42136829/whats-difference-between-http-301-and-308-status-codes

Helpers.DownloadAsync lines 54 (new) and 55 (changed):

var statusCode = (int)response.StatusCode; if (autoRedirect && (statusCode == 301 || statusCode == 308)) // moved

Example returning a 301 status code: http://www.medicalnewstoday.com/rss/diabetes.xml

[arminreiter]

Hi!
I fixed an encoding issue today and with this issue, I also added support for 301 status code: see:
bd78248