rewrite the setup of responses to use a standard helper function (#2177)

Octokit/Http/Response.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Net;
 
@@ -9,10 +10,12 @@ namespace Octokit.Internal
     /// </summary>
     internal class Response : IResponse
     {
+        [Obsolete("Use the constuctor with maximum parameters to avoid shortcuts")]
         public Response() : this(new Dictionary<string, string>())
         {
         }
 
+        [Obsolete("Use the constuctor with maximum parameters to avoid shortcuts")]
         public Response(IDictionary<string, string> headers)
         {
             Ensure.ArgumentNotNull(headers, nameof(headers));
@@ -53,4 +56,4 @@ namespace Octokit.Internal
         /// </summary>
         public string ContentType { get; private set; }
     }
-}
+}