Replace all other uses of hardcoded HTTP header strings to declare/use entries in the AcceptHeaders helper class

Left test methods alone as it isn't good practice to share these definitions between Test and Implementation classes

Octokit/Http/JsonHttpPipeline.cs

@@ -11,7 +11,7 @@ namespace Octokit.Internal
     /// </summary>
     public class JsonHttpPipeline
     {
-        private const string v3ApiVersion = "application/vnd.github.quicksilver-preview+json; charset=utf-8, application/vnd.github.v3+json; charset=utf-8";
+        private const string v3ApiVersion = AcceptHeaders.DefaultJson;
 
         readonly IJsonSerializer _serializer;