remove nullable strings from RequestOptions

src/Discord.Net.Core/RequestOptions.cs

@@ -71,7 +71,7 @@ namespace Discord
         internal bool IsReactionBucket { get; set; }
         internal bool IsGatewayBucket { get; set; }
 
-        internal IDictionary<string, IEnumerable<string?>> RequestHeaders { get; }
+        internal IDictionary<string, IEnumerable<string>> RequestHeaders { get; }
 
         internal static RequestOptions CreateOrClone(RequestOptions options)
         {
@@ -99,7 +99,7 @@ namespace Discord
         public RequestOptions()
         {
             Timeout = DiscordConfig.DefaultRequestTimeout;
-            RequestHeaders = new Dictionary<string, IEnumerable<string?>>();
+            RequestHeaders = new Dictionary<string, IEnumerable<string>>();
         }
 
         public RequestOptions Clone() => MemberwiseClone() as RequestOptions;