Feature: Disconnect users from voice channels

Updates GuildUserProperties to allow for setting either Channel or ChannelId to null, which will disconnect users from voice channels.

The type of ChannelId has been updated from a ulong to ulong?, which matches the latest api documentation.

src/Discord.Net.Core/Entities/Users/GuildUserProperties.cs

@@ -59,18 +59,19 @@ namespace Discord
         /// </remarks>
         public Optional<IEnumerable<ulong>> RoleIds { get; set; }
         /// <summary>
-        ///     Moves a user to a voice channel.
+        ///     Moves a user to a voice channel. If <c>null</c>, this user will be disconnected from voice.
         /// </summary>
         /// <remarks>
         ///     This user MUST already be in a <see cref="IVoiceChannel"/> for this to work.
+        ///     When set, this property takes precedence over <see cref="ChannelId"/>.
         /// </remarks>
         public Optional<IVoiceChannel> Channel { get; set; }
         /// <summary>
-        ///     Moves a user to a voice channel.
+        ///     Moves a user to a voice channel. If <c>null</c>, this user will be disconnected from voice.
         /// </summary>
         /// <remarks>
         ///     This user MUST already be in a <see cref="IVoiceChannel"/> for this to work.
         /// </remarks>
-        public Optional<ulong> ChannelId { get; set; }
+        public Optional<ulong?> ChannelId { get; set; }
     }
 }

src/Discord.Net.Rest/API/Rest/ModifyGuildMemberParams.cs

@@ -1,4 +1,4 @@
-#pragma warning disable CS1591
+#pragma warning disable CS1591
 using Newtonsoft.Json;
 
 namespace Discord.API.Rest
@@ -15,6 +15,6 @@ namespace Discord.API.Rest
         [JsonProperty("roles")]
         public Optional<ulong[]> RoleIds { get; set; }
         [JsonProperty("channel_id")]
-        public Optional<ulong> ChannelId { get; set; }
+        public Optional<ulong?> ChannelId { get; set; }
     }
 }

src/Discord.Net.Rest/Entities/Users/UserHelper.cs

@@ -1,4 +1,4 @@
-using Discord.API.Rest;
+using Discord.API.Rest;
 using System;
 using System.Collections.Generic;
 using System.Threading.Tasks;
@@ -39,7 +39,7 @@ namespace Discord.Rest
             };
 
             if (args.Channel.IsSpecified)
-                apiArgs.ChannelId = args.Channel.Value.Id;
+                apiArgs.ChannelId = args.Channel.Value?.Id;
             else if (args.ChannelId.IsSpecified)
                 apiArgs.ChannelId = args.ChannelId.Value;