Use ToString in converter instead of boxing-cast

In cases where Discord sent a value of `id=0`, this would throw an invalid-cast, where 0u64 cannot be cast to string.

src/Discord.Net.Core/Net/Converters/NullableUInt64Converter.cs

@@ -16,7 +16,7 @@ namespace Discord.Net.Converters
         {
             object value = reader.Value;
             if (value != null)
-                return ulong.Parse((string)value, NumberStyles.None, CultureInfo.InvariantCulture);
+                return ulong.Parse(value.ToString(), NumberStyles.None, CultureInfo.InvariantCulture);
             else
                 return null;
         }