Fix race that would allow GetDateHeaderValues to return null

Author: halter73

src/Microsoft.AspNetCore.Server.Kestrel/Http/DateHeaderValueManager.cs

@@ -58,9 +58,11 @@ internal DateHeaderValueManager(
         /// <returns>The value in string and byte[] format.</returns>
         public DateHeaderValues GetDateHeaderValues()
         {
-            if (!_hadRequestsSinceLastTimerTick)
+            _hadRequestsSinceLastTimerTick = !_isDisposed;
+
+            if (!_timerIsRunning)
             {
-                PrepareDateValues();
+                StartTimer();
             }
 
             return _dateValues;
@@ -151,18 +153,6 @@ private void TimerLoop(object state)
             }
         }
 
-        /// <summary>
-        /// Starts the timer if it's turned off, or sets the datevalues to the current time if disposed. 
-        /// </summary>
-        private void PrepareDateValues()
-        {
-            _hadRequestsSinceLastTimerTick = !_isDisposed;
-            if (!_timerIsRunning)
-            {
-                StartTimer();
-            }
-        }
-
         /// <summary>
         /// Sets date values from a provided ticks value
         /// </summary>