Convert method to static that doesn't use instance data

(cherry picked from commit a42f97229acb713719c616851db572100f319ad7)

src/NzbDrone.Common/Http/Dispatchers/ManagedHttpDispatcher.cs

@@ -240,7 +240,7 @@ namespace NzbDrone.Common.Http.Dispatchers
             }
         }
 
-        private void AddContentHeader(HttpRequestMessage request, string header, string value)
+        private static void AddContentHeader(HttpRequestMessage request, string header, string value)
         {
             var headers = request.Content?.Headers;
             if (headers == null)