Introduce emoji role property into rolehelper (#360)

* Introduce emoji role property into rolehelper

* Add check for exclusive properties

* Oversight resolved

* Valid order for error responses

* Appending suggestions

src/Discord.Net.Core/Entities/Roles/RoleProperties.cs

@@ -1,3 +1,5 @@
+using System;
+
 namespace Discord
 {
     /// <summary>
@@ -50,11 +52,24 @@ namespace Discord
         ///     This value may not be set if the role is an @everyone role.
         /// </remarks>
         public Optional<bool> Hoist { get; set; }
-
         /// <summary>
         ///     Gets or sets the icon of the role.
         /// </summary>
-        public Optional<Image> Icon { get; set; }
+        /// <remarks>
+        ///     This value cannot be set at the same time as Emoji, as they are both exclusive.
+        ///     
+        ///     Setting an Icon will override a currently existing Emoji if present.
+        /// </remarks>
+        public Optional<Image?> Icon { get; set; }
+        /// <summary>
+        ///     Gets or sets the unicode emoji of the role.
+        /// </summary>
+        /// <remarks>
+        ///     This value cannot be set at the same time as Icon, as they are both exclusive.
+        ///
+        ///     Setting an Emoji will override a currently existing Icon if present.
+        /// </remarks>
+        public Optional<Emoji> Emoji { get; set; }
         /// <summary>
         ///     Gets or sets whether or not this role can be mentioned.
         /// </summary>

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

@@ -14,7 +14,9 @@ namespace Discord.API.Rest
         [JsonProperty("hoist")]
         public Optional<bool> Hoist { get; set; }
         [JsonProperty("icon")]
-        public Optional<Image> Icon { get; set; }
+        public Optional<Image?> Icon { get; set; }
+        [JsonProperty("unicode_emoji")]
+        public Optional<string> Emoji { get; set; }
         [JsonProperty("mentionable")]
         public Optional<bool> Mentionable { get; set; }
     }

src/Discord.Net.Rest/Entities/Roles/RoleHelper.cs

@@ -13,15 +13,20 @@ namespace Discord.Rest
         {
             await client.ApiClient.DeleteGuildRoleAsync(role.Guild.Id, role.Id, options).ConfigureAwait(false);
         }
-        public static async Task<Model> ModifyAsync(IRole role, BaseDiscordClient client, 
+        public static async Task<Model> ModifyAsync(IRole role, BaseDiscordClient client,
             Action<RoleProperties> func, RequestOptions options)
         {
             var args = new RoleProperties();
             func(args);
 
-            if (args.Icon.IsSpecified)
+            if (args.Icon.IsSpecified || args.Emoji.IsSpecified)
             {
                 role.Guild.Features.EnsureFeature(GuildFeature.RoleIcons);
+
+                if (args.Icon.IsSpecified && args.Emoji.IsSpecified)
+                {
+                    throw new ArgumentException("Emoji and Icon properties cannot be present on a role at the same time.");
+                }
             }
 
             var apiArgs = new API.Rest.ModifyGuildRoleParams
@@ -31,8 +36,20 @@ namespace Discord.Rest
                 Mentionable = args.Mentionable,
                 Name = args.Name,
                 Permissions = args.Permissions.IsSpecified ? args.Permissions.Value.RawValue.ToString() : Optional.Create<string>(),
-                Icon = args.Icon.IsSpecified ? args.Icon.Value.ToModel() : Optional<API.Image>.Unspecified
+                Icon = args.Icon.IsSpecified ? args.Icon.Value.Value.ToModel() : Optional<API.Image?>.Unspecified,
+                Emoji = args.Emoji.GetValueOrDefault()?.Name ?? Optional<string>.Unspecified
             };
+
+            if (args.Icon.IsSpecified && role.Emoji != null)
+            {
+                apiArgs.Emoji = null;
+            }
+
+            if (args.Emoji.IsSpecified && !string.IsNullOrEmpty(role.Icon))
+            {
+                apiArgs.Icon = null;
+            }
+
             var model = await client.ApiClient.ModifyGuildRoleAsync(role.Guild.Id, role.Id, apiArgs, options).ConfigureAwait(false);
 
             if (args.Position.IsSpecified)