UnixTimestampConverter should now obey a maximum value

This change prevents an issue where the converter would be unable to
handle obscenely large timestamp values - which are actually quite
common on Discord.

OptionalConverter had to be rewritten to support checking whether or not
an InnerConverter returned an Optional. The perf impacts from this
_shouldn't_ be too bad, as types without a custom parser (which should
be the majority of Optionals in the lib) will bypass the type-check.

src/Discord.Net.Rest/Net/Converters/OptionalConverter.cs

@@ -1,4 +1,4 @@
-using Newtonsoft.Json;
+using Newtonsoft.Json;
 using System;
 
 namespace Discord.Net.Converters
@@ -18,12 +18,15 @@ namespace Discord.Net.Converters
 
         public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
         {
-            T obj;
+            object obj;
             if (_innerConverter != null)
-                obj = (T)_innerConverter.ReadJson(reader, typeof(T), null, serializer);
+                obj = _innerConverter.ReadJson(reader, typeof(T), null, serializer);
             else
                 obj = serializer.Deserialize<T>(reader);
-            return new Optional<T>(obj);
+
+            if (obj is Optional<T>)
+                return obj;
+            return new Optional<T>((T)obj);
         }
 
         public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)

src/Discord.Net.Rest/Net/Converters/UnixTimestampConverter.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using Newtonsoft.Json;
 
 namespace Discord.Net.Converters
@@ -11,13 +11,18 @@ namespace Discord.Net.Converters
         public override bool CanRead => true;
         public override bool CanWrite => true;
 
+        // 1e13 unix ms = year 2286
+        // necessary to prevent discord.js from sending values in the e15 and overflowing a DTO
+        private const long MaxSaneMs = 1_000_000_000_000_0;
+
         public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
         {
-            // Discord doesn't validate if timestamps contain decimals or not
-            if (reader.Value is double d)
+            // Discord doesn't validate if timestamps contain decimals or not, and they also don't validate if timestamps are reasonably sized
+            if (reader.Value is double d && d < MaxSaneMs)
                 return new DateTimeOffset(1970, 1, 1, 0, 0, 0, 0, TimeSpan.Zero).AddMilliseconds(d);
-            long offset = (long)reader.Value;
-            return new DateTimeOffset(1970, 1, 1, 0, 0, 0, 0, TimeSpan.Zero).AddMilliseconds(offset);
+            else if (reader.Value is long l && l < MaxSaneMs)
+                return new DateTimeOffset(1970, 1, 1, 0, 0, 0, 0, TimeSpan.Zero).AddMilliseconds(l);
+            return Optional<DateTimeOffset>.Unspecified;
         }
 
         public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
@@ -25,4 +30,4 @@ namespace Discord.Net.Converters
             throw new NotImplementedException();
         }
     }
-}
+}