Added RFC 7538 support

Author: 0xdeafcafe

src/Microsoft.AspNetCore.Http.Abstractions/StatusCodes.cs

@@ -22,6 +22,7 @@ public static class StatusCodes
         public const int Status305UseProxy = 305;
         public const int Status306SwitchProxy = 306;
         public const int Status307TemporaryRedirect = 307;
+        public const int Status308PermanentRedirect = 308;
 
         public const int Status400BadRequest = 400;
         public const int Status401Unauthorized = 401;