fix: AllowedMentions and AllowedMentionTypes (#1525)

* Give proper values to flag enum

* Add zero value

* Initialize lists

* Update xml docs

src/Discord.Net.Core/Entities/Messages/AllowedMentionTypes.cs

@@ -9,16 +9,26 @@ namespace Discord
     public enum AllowedMentionTypes
     {
         /// <summary>
+        ///     No flag is set.
+        /// </summary>
+        /// <remarks>
+        ///     This flag is not used to control mentions.
+        ///     <note type="warning">
+        ///         It will always be present and does not mean mentions will not be allowed.
+        ///     </note>
+        /// </remarks>
+        None        = 0,
+        /// <summary>
         ///     Controls role mentions.
         /// </summary>
-        Roles,
+        Roles       = 1,
         /// <summary>
         ///     Controls user mentions.
         /// </summary>
-        Users,
+        Users       = 2,
         /// <summary>
         ///     Controls <code>@everyone</code> and <code>@here</code> mentions.
         /// </summary>
-        Everyone,
+        Everyone    = 4,
     }
 }

src/Discord.Net.Core/Entities/Messages/AllowedMentions.cs

@@ -39,7 +39,7 @@ namespace Discord
         ///     flag of the <see cref="AllowedTypes"/> property. If the flag is set, the value of this property
         ///     must be <c>null</c> or empty.
         /// </summary>
-        public List<ulong> RoleIds { get; set; }
+        public List<ulong> RoleIds { get; set; } = new List<ulong>();
 
         /// <summary>
         ///     Gets or sets the list of all user ids that will be mentioned.
@@ -47,7 +47,7 @@ namespace Discord
         ///     flag of the <see cref="AllowedTypes"/> property. If the flag is set, the value of this property
         ///     must be <c>null</c> or empty.
         /// </summary>
-        public List<ulong> UserIds { get; set; }
+        public List<ulong> UserIds { get; set; } = new List<ulong>();
 
         /// <summary>
         ///     Initializes a new instance of the <see cref="AllowedMentions"/> class.