Make IResponse.Headers a readonly dictionary

Octokit/Http/ApiResponse.cs

@@ -1,6 +1,4 @@
-using System;
-using System.Collections.Generic;
-using System.Collections.ObjectModel;
+using System.Collections.Generic;
 using System.Net;
 
 namespace Octokit.Internal
@@ -17,7 +15,7 @@ namespace Octokit.Internal
 
             var body = response.Body is T ? (T)response.Body : default(T);
             HttpResponse = response;
-            Headers = new ReadOnlyDictionary<string, string>(response.Headers);
+            Headers = response.Headers;
             Body = body;
             ApiInfo = response.ApiInfo;
             StatusCode = response.StatusCode;

Octokit/Http/IResponse.cs

@@ -18,7 +18,7 @@ namespace Octokit
     public interface IResponse
     {
         object Body { get; }
-        IDictionary<string, string> Headers { get; }
+        IReadOnlyDictionary<string, string> Headers { get; }
         ApiInfo ApiInfo { get; }
         HttpStatusCode StatusCode { get; }
         string ContentType { get; }

Octokit/Http/Response.cs

@@ -1,8 +1,6 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Net;
-using System.Net.Http;
 using Octokit.Internal;
 
 namespace Octokit.Internal
@@ -17,7 +15,7 @@ namespace Octokit.Internal
         {
             Ensure.ArgumentNotNull(headers, "headers");
 
-            Headers = headers;
+            Headers = new ReadOnlyDictionary<string, string>(headers);
             ApiInfo = ApiInfoParser.ParseResponseHeaders(headers);
         }
 
@@ -27,13 +25,13 @@ namespace Octokit.Internal
 
             StatusCode = statusCode;
             Body = body;
-            Headers = headers;
+            Headers = new ReadOnlyDictionary<string, string>(headers);
             ApiInfo = ApiInfoParser.ParseResponseHeaders(headers);
             ContentType = contentType;
         }
 
         public object Body { get; set; }
-        public IDictionary<string, string> Headers { get; private set; }
+        public IReadOnlyDictionary<string, string> Headers { get; private set; }
         public ApiInfo ApiInfo { get; private set; }
         public HttpStatusCode StatusCode { get; set; }
         public string ContentType { get; private set; }