fix: add EmbedTypeConverter with unknown handling

Prevents messages with an unknown EmbedType from failing to unmarshal

src/Discord.Net.Rest/API/Common/Embed.cs

@@ -1,7 +1,7 @@
-#pragma warning disable CS1591
+#pragma warning disable CS1591
 using System;
 using Newtonsoft.Json;
-using Newtonsoft.Json.Converters;
+using Discord.Net.Converters;
 
 namespace Discord.API
 {
@@ -15,7 +15,7 @@ namespace Discord.API
         public string Url { get; set; }
         [JsonProperty("color")]
         public uint? Color { get; set; }
-        [JsonProperty("type"), JsonConverter(typeof(StringEnumConverter))]
+        [JsonProperty("type"), JsonConverter(typeof(EmbedTypeConverter))]
         public EmbedType Type { get; set; }
         [JsonProperty("timestamp")]
         public DateTimeOffset? Timestamp { get; set; }

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

@@ -77,6 +77,8 @@ namespace Discord.Net.Converters
                 return PermissionTargetConverter.Instance;
             if (type == typeof(UserStatus))
                 return UserStatusConverter.Instance;
+            if (type == typeof(EmbedType))
+                return EmbedTypeConverter.Instance;
 
             //Special
             if (type == typeof(API.Image))

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

@@ -0,0 +1,73 @@
+using System;
+using Newtonsoft.Json;
+
+namespace Discord.Net.Converters
+{
+    internal class EmbedTypeConverter : JsonConverter
+    {
+        public static readonly EmbedTypeConverter Instance = new EmbedTypeConverter();
+
+        public override bool CanConvert(Type objectType) => true;
+        public override bool CanRead => true;
+        public override bool CanWrite => true;
+
+        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        {
+            switch ((string)reader.Value)
+            {
+                case "rich":
+                    return EmbedType.Rich;
+                case "link":
+                    return EmbedType.Link;
+                case "video":
+                    return EmbedType.Video;
+                case "image":
+                    return EmbedType.Image;
+                case "gifv":
+                    return EmbedType.Gifv;
+                case "article":
+                    return EmbedType.Article;
+                case "tweet":
+                    return EmbedType.Tweet;
+                case "html":
+                    return EmbedType.Html;
+                case "application_news": // TODO 2.2 EmbedType.News
+                default:
+                    return EmbedType.Unknown;
+            }
+        }
+
+        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        {
+            switch ((EmbedType)value)
+            {
+                case EmbedType.Rich:
+                    writer.WriteValue("rich");
+                    break;
+                case EmbedType.Link:
+                    writer.WriteValue("link");
+                    break;
+                case EmbedType.Video:
+                    writer.WriteValue("video");
+                    break;
+                case EmbedType.Image:
+                    writer.WriteValue("image");
+                    break;
+                case EmbedType.Gifv:
+                    writer.WriteValue("gifv");
+                    break;
+                case EmbedType.Article:
+                    writer.WriteValue("article");
+                    break;
+                case EmbedType.Tweet:
+                    writer.WriteValue("tweet");
+                    break;
+                case EmbedType.Html:
+                    writer.WriteValue("html");
+                    break;
+                default:
+                    throw new JsonSerializationException("Invalid embed type");
+            }
+        }
+    }
+}