Added new let constant to Request ’s timeline variable. (#2125)

Source/ResponseSerialization.swift

@@ -88,11 +88,12 @@ public struct DownloadResponseSerializer<Value>: DownloadResponseSerializerProto
 
 extension Request {
     var timeline: Timeline {
+        let requestStartTime = self.startTime ?? CFAbsoluteTimeGetCurrent()
         let requestCompletedTime = self.endTime ?? CFAbsoluteTimeGetCurrent()
         let initialResponseTime = self.delegate.initialResponseTime ?? requestCompletedTime
 
         return Timeline(
-            requestStartTime: self.startTime ?? CFAbsoluteTimeGetCurrent(),
+            requestStartTime: requestStartTime,
             initialResponseTime: initialResponseTime,
             requestCompletedTime: requestCompletedTime,
             serializationCompletedTime: CFAbsoluteTimeGetCurrent()