Make IResponse.ContentType readonly

Octokit/Http/Response.cs

@@ -1,6 +1,8 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Net;
+using System.Net.Http;
 using Octokit.Internal;
 
 namespace Octokit.Internal
@@ -19,10 +21,21 @@ namespace Octokit.Internal
             ApiInfo = ApiInfoParser.ParseResponseHeaders(headers);
         }
 
+        public Response(HttpStatusCode statusCode, object body, IDictionary<string, string> headers, string contentType)
+        {
+            Ensure.ArgumentNotNull(headers, "headers");
+
+            StatusCode = statusCode;
+            Body = body;
+            Headers = headers;
+            ApiInfo = ApiInfoParser.ParseResponseHeaders(headers);
+            ContentType = contentType;
+        }
+
         public object Body { get; set; }
         public IDictionary<string, string> Headers { get; private set; }
         public ApiInfo ApiInfo { get; private set; }
         public HttpStatusCode StatusCode { get; set; }
-        public string ContentType { get; set; }
+        public string ContentType { get; private set; }
     }
 }