Actually use GetMessages(beforeMessageId)

src/Discord.Net/API/Endpoints.cs

@@ -16,6 +16,7 @@
 		public static string ChannelTyping(string channelId) => $"channels/{channelId}/typing";
 		public static string ChannelMessages(string channelId) => $"channels/{channelId}/messages";
 		public static string ChannelMessages(string channelId, int limit) => $"channels/{channelId}/messages?limit={limit}";
+		public static string ChannelMessages(string channelId, int limit, string beforeId) => $"channels/{channelId}/messages?limit={limit}&before={beforeId}";
 		public static string ChannelMessage(string channelId, string msgId) => $"channels/{channelId}/messages/{msgId}";
 		public static string ChannelMessageAck(string channelId, string msgId) => $"channels/{channelId}/messages/{msgId}/ack";
 		public static string ChannelInvites(string channelId) => $"channels/{channelId}/invites";

src/Discord.Net/DiscordAPIClient.cs

@@ -92,11 +92,14 @@ namespace Discord
 			var request = new ReorderChannelsRequest(channels);
 			return _rest.Patch(Endpoints.ServerChannels(serverId), request);
 		}
-		public Task<GetMessagesResponse> GetMessages(string channelId, int count)
+		public Task<GetMessagesResponse> GetMessages(string channelId, int count, string beforeMessageId = null)
 		{
 			if (channelId == null) throw new ArgumentNullException(nameof(channelId));
 
-			return _rest.Get<GetMessagesResponse>(Endpoints.ChannelMessages(channelId, count));
+			if (beforeMessageId != null)
+				return _rest.Get<GetMessagesResponse>(Endpoints.ChannelMessages(channelId, count, beforeMessageId));
+			else
+				return _rest.Get<GetMessagesResponse>(Endpoints.ChannelMessages(channelId, count));
 		}
 
 		//Incidents

src/Discord.Net/DiscordClient.Messages.cs

@@ -210,7 +210,7 @@ namespace Discord
 			{
 				try
 				{
-					var msgs = await _api.GetMessages(channel.Id, count).ConfigureAwait(false);
+					var msgs = await _api.GetMessages(channel.Id, count, beforeMessageId).ConfigureAwait(false);
 					return msgs.Select(x =>
 					{
 						Message msg = null;