fix: RestDMChannel#SendMessageAsync bug, allowed mentions always null

This change fixes a bug that was introduced while testing changes to the interface of the SendMessageAsync method to try and retain interface compatibility

src/Discord.Net.Rest/Entities/Channels/RestDMChannel.cs

@@ -94,7 +94,7 @@ namespace Discord.Rest
         /// <inheritdoc />
         /// <exception cref="ArgumentOutOfRangeException">Message content is too long, length must be less or equal to <see cref="DiscordConfig.MaxMessageSize"/>.</exception>
         public Task<RestUserMessage> SendMessageAsync(string text = null, bool isTTS = false, Embed embed = null, RequestOptions options = null, AllowedMentions allowedMentions = null)
-            => ChannelHelper.SendMessageAsync(this, Discord, text, isTTS, embed, null, options);
+            => ChannelHelper.SendMessageAsync(this, Discord, text, isTTS, embed, allowedMentions, options);
 
         /// <inheritdoc />
         /// <exception cref="ArgumentException">