While RFC 822 and RFC 1123 both allow either one- or two-digit days in dates, HTTP only allows two-digit days (see http://tools.ietf.org/html/rfc2616#section-3.3.1)
Mono currently outputs dates in this format:
Sun, 3 Mar 2014 13:22:27 GMT
When they should be in this format:
Sun, 03 Mar 2014 13:22:27 GMT
I've fixed this and will submit the fix via Github; this bug is just for tracking purposes.
Pull request submitted: https://github.com/mono/mono/pull/588