Revert "remove unnecessary usings"

This reverts commit 8d674fe4fa.

src/Discord.Net.Core/RequestOptions.cs

@@ -70,6 +70,7 @@ namespace Discord
         internal bool IsClientBucket { get; set; }
         internal bool IsReactionBucket { get; set; }
         internal bool IsGatewayBucket { get; set; }
+
         internal IDictionary<string, IEnumerable<string?>> RequestHeaders { get; }
 
         internal static RequestOptions CreateOrClone(RequestOptions options)

src/Discord.Net.Rest/Net/Queue/Requests/RestRequest.cs

@@ -1,5 +1,8 @@
 using Discord.Net.Rest;
 using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.IO;
 using System.Threading.Tasks;