fix PremiumSubscriptionCount optional value

src/Discord.Net.Rest/API/Common/Guild.cs

@@ -57,6 +57,6 @@ namespace Discord.API
         [JsonProperty("system_channel_flags")]
         public SystemChannelMessageDeny SystemChannelFlags { get; set; }
         [JsonProperty("premium_subscription_count")]
-        public Optional<int> PremiumSubscriptionCount { get; set; }
+        public int? PremiumSubscriptionCount { get; set; }
     }
 }

src/Discord.Net.Rest/Entities/Guilds/RestGuild.cs

@@ -123,8 +123,7 @@ namespace Discord.Rest
             BannerId = model.Banner;
             SystemChannelFlags = model.SystemChannelFlags;
             Description = model.Description;
-            if (model.PremiumSubscriptionCount.IsSpecified)
-                PremiumSubscriptionCount = model.PremiumSubscriptionCount.Value;
+            PremiumSubscriptionCount = model.PremiumSubscriptionCount.GetValueOrDefault();
 
             if (model.Emojis != null)
             {