Included LastModified in response headers

Implemented logic to keep record of the timestamp when a response content was generated and use it as the Last-Modified response header

Author: dmalanij

src/WebApi.OutputCache.Core/Constants.cs

@@ -4,5 +4,6 @@ public sealed class Constants
     {
         public const string ContentTypeKey = ":response-ct";
         public const string EtagKey = ":response-etag";
+        public const string GenerationTimestampKey = ":response-generationtimestamp";
     }
 }

src/WebApi.OutputCache.V2/CacheOutputAttribute.cs

@@ -195,6 +195,7 @@ public override void OnActionExecuting(HttpActionContext actionContext)
             if (val == null) return;
 
             var contenttype = _webApiCache.Get<MediaTypeHeaderValue>(cachekey + Constants.ContentTypeKey) ?? responseMediaType;
+            var contentGenerationTimestamp = DateTimeOffset.Parse(_webApiCache.Get<string>(cachekey + Constants.GenerationTimestampKey));
 
             actionContext.Response = actionContext.Request.CreateResponse();
             actionContext.Response.Content = new ByteArrayContent(val);
@@ -204,7 +205,7 @@ public override void OnActionExecuting(HttpActionContext actionContext)
             if (responseEtag != null) SetEtag(actionContext.Response,  responseEtag);
 
             var cacheTime = CacheTimeQuery.Execute(DateTime.Now);
-            ApplyCacheHeaders(actionContext.Response, cacheTime);
+            ApplyCacheHeaders(actionContext.Response, cacheTime, contentGenerationTimestamp);
         }
 
         public override async Task OnActionExecutedAsync(HttpActionExecutedContext actionExecutedContext, CancellationToken cancellationToken)
@@ -213,8 +214,9 @@ public override async Task OnActionExecutedAsync(HttpActionExecutedContext actio
 
             if (!IsCachingAllowed(actionExecutedContext.ActionContext, AnonymousOnly)) return;
 
-            var cacheTime = CacheTimeQuery.Execute(DateTime.Now);
-            if (cacheTime.AbsoluteExpiration > DateTime.Now)
+            var actionExecutionTimestamp = DateTimeOffset.Now;
+            var cacheTime = CacheTimeQuery.Execute(actionExecutionTimestamp.DateTime);
+            if (cacheTime.AbsoluteExpiration > actionExecutionTimestamp)
             {
                 var httpConfig = actionExecutedContext.Request.GetConfiguration();
                 var config = httpConfig.CacheOutputConfiguration();
@@ -251,14 +253,19 @@ public override async Task OnActionExecutedAsync(HttpActionExecutedContext actio
                         _webApiCache.Add(cachekey + Constants.EtagKey,
                                         etag,
                                         cacheTime.AbsoluteExpiration, baseKey);
+
+
+                        _webApiCache.Add(cachekey + Constants.GenerationTimestampKey,
+                                        actionExecutionTimestamp.ToString(),
+                                        cacheTime.AbsoluteExpiration, baseKey);
                     }
                 }
             }
 
-            ApplyCacheHeaders(actionExecutedContext.ActionContext.Response, cacheTime);
+            ApplyCacheHeaders(actionExecutedContext.ActionContext.Response, cacheTime, actionExecutionTimestamp);
         }
 
-        protected virtual void ApplyCacheHeaders(HttpResponseMessage response, CacheTime cacheTime)
+        protected virtual void ApplyCacheHeaders(HttpResponseMessage response, CacheTime cacheTime, DateTimeOffset? contentGenerationTimestamp = null)
         {
             if (cacheTime.ClientTimeSpan > TimeSpan.Zero || MustRevalidate || Private)
             {
@@ -277,6 +284,10 @@ protected virtual void ApplyCacheHeaders(HttpResponseMessage response, CacheTime
                 response.Headers.CacheControl = new CacheControlHeaderValue { NoCache = true };
                 response.Headers.Add("Pragma", "no-cache");
             }
+            if ((response.Content != null) && contentGenerationTimestamp.HasValue)
+            {
+                response.Content.Headers.LastModified = contentGenerationTimestamp.Value;
+            }
         }
 
         protected virtual string CreateEtag(HttpActionExecutedContext actionExecutedContext, string cachekey, CacheTime cacheTime)