Fix MaxDate.ToUnixTime() Issue

Author: mythz

src/ServiceStack.Text/DateTimeExtensions.cs

@@ -62,7 +62,7 @@ private static TimeSpan ToDateTimeSinceUnixEpoch(this DateTime dateTime)
             var dtUtc = dateTime;
             if (dateTime.Kind != DateTimeKind.Utc)
             {
-                dtUtc = dateTime.Kind == DateTimeKind.Unspecified && dateTime > DateTime.MinValue
+                dtUtc = dateTime.Kind == DateTimeKind.Unspecified && dateTime > DateTime.MinValue && dateTime < DateTime.MaxValue
                     ? DateTime.SpecifyKind(dateTime.Subtract(DateTimeSerializer.LocalTimeZone.GetUtcOffset(dateTime)), DateTimeKind.Utc)
                     : dateTime.ToStableUniversalTime();
             }

tests/ServiceStack.Text.Tests/Utils/DateTimeSerializerTests.cs

@@ -297,6 +297,13 @@ public void Test_MS_Dates(int whichDate)
 
             AssertDatesAreEqual(fromStr, dateTime);
         }
+
+        [Test]
+        public void Can_serialize_MaxDateTime()
+        {
+            var maxDate = DateTime.MaxValue.ToUnixTime();
+            var minDate = DateTime.MinValue.ToUnixTime();
+        }
     }
 
     public class DateModel