Change HttpRequestAction activity to store headers in dictionary

This fixes a deserialization issue with MongoDb that cannot deserialize StringValues.

src/activities/Elsa.Activities.Http/Activities/HttpRequestAction.cs

@@ -159,8 +159,8 @@ namespace Elsa.Activities.Http.Activities
             var responseModel = new HttpResponseModel
             {
                 StatusCode = response.StatusCode,
-                Headers = new HeaderDictionary(
-                    response.Headers.ToDictionary(x => x.Key, x => new StringValues(x.Value.ToArray()))
+                Headers = new Dictionary<string, string[]>(
+                    response.Headers.ToDictionary(x => x.Key, x => x.Value.ToArray())
                 )
             };
 

src/activities/Elsa.Activities.Http/Models/HttpResponseModel.cs

@@ -1,5 +1,5 @@
+using System.Collections.Generic;
 using System.Net;
-using Microsoft.AspNetCore.Http;
 
 namespace Elsa.Activities.Http.Models
 {
@@ -10,7 +10,7 @@ namespace Elsa.Activities.Http.Models
         }
 
         public HttpStatusCode StatusCode { get; set; }
-        public IHeaderDictionary Headers { get; set; } = new HeaderDictionary();
+        public Dictionary<string, string[]> Headers { get; set; } = new Dictionary<string, string[]>();
         public byte[] Content { get; set; }
         public object ParsedContent { get; set; }
     }

src/activities/Elsa.Activities.Timers/Activities/TimerEvent.cs

@@ -66,9 +66,7 @@ namespace Elsa.Activities.Timers.Activities
             var now = clock.GetCurrentInstant();
 
             if (StartTime == null)
-            {
                 StartTime = now;
-            }
             
             var timeSpan = await expressionEvaluator.EvaluateAsync(TimeoutExpression, context, cancellationToken);
             var expiresAt = StartTime.Value.ToDateTimeUtc() + timeSpan;