FormattingUtilities support for parsing dates in RFC 850 long year format (#196)

- also add RFC 850 long year format related test case

src/System.Net.Http.Formatting/FormattingUtilities.cs

@@ -30,8 +30,9 @@ namespace System.Net.Http
             "d MMM yy H:m:s 'GMT'", // RFC 1123, no day-of-week, short year
             "d MMM yy H:m:s", // RFC 1123, no day-of-week, short year, no zone
 
-            "dddd, d'-'MMM'-'yy H:m:s 'GMT'", // RFC 850
+            "dddd, d'-'MMM'-'yy H:m:s 'GMT'", // RFC 850, short year
             "dddd, d'-'MMM'-'yy H:m:s", // RFC 850 no zone
+            "ddd, d'-'MMM'-'yyyy H:m:s 'GMT'", // RFC 850, long year
             "ddd MMM d H:m:s yyyy", // ANSI C's asctime() format
 
             "ddd, d MMM yyyy H:m:s zzz", // RFC 5322

test/System.Net.Http.Formatting.Test/FormattingUtilitiesTests.cs

@@ -100,6 +100,7 @@ namespace System.Net.Http
                     // RFC850 date/time value
                     { "Sunday, 06-Nov-94 08:49:37 GMT",  new DateTimeOffset(1994, 11, 6, 8, 49, 37, TimeSpan.Zero) },
                     { "Sunday, 6-Nov-94 8:49:37",  new DateTimeOffset(1994, 11, 6, 8, 49, 37, TimeSpan.Zero) },
+                    { "Sun, 6-Nov-1994 8:49:37 GMT", new DateTimeOffset(1994, 11, 6, 8, 49, 37, TimeSpan.Zero) },
 
                     // ANSI C's asctime() format
                     { "Sun Nov  06 08:49:37 1994",  new DateTimeOffset(1994, 11, 6, 8, 49, 37, TimeSpan.Zero) },