Fixed channel topics

src/Discord.Net/API/Common.cs

@@ -109,7 +109,7 @@ namespace Discord.API
 		[JsonProperty("is_private")]
 		public bool IsPrivate;
 		[JsonProperty("position")]
-		public int Position;
+		public int? Position;
 		[JsonProperty(PropertyName = "topic")]
 		public string Topic;
 		[JsonProperty("permission_overwrites")]

src/Discord.Net/Models/Channel.cs

@@ -89,14 +89,19 @@ namespace Discord
 
 		internal void Update(API.ChannelReference model)
 		{
-			Name = model.Name;
-			Type = model.Type;
+			if (model.Name != null)
+				Name = model.Name;
+			if (model.Type != null)
+				Type = model.Type;
 		}
 		internal void Update(API.ChannelInfo model)
 		{
 			Update(model as API.ChannelReference);
 			
-			Position = model.Position;
+			if (model.Position != null)
+				Position = model.Position.Value;
+			if (model.Topic != null)
+				Topic = model.Topic;
 
 			if (model.PermissionOverwrites != null)
 			{