fix(visual-recognition-v4): add DateTime format

Author: kevinkowa

Scripts/Services/VisualRecognition/V4/VisualRecognitionService.cs

@@ -1682,11 +1682,11 @@ public bool GetTrainingUsage(Callback<TrainingEvents> callback, DateTime? startT
             }
             if (startTime != null)
             {
-                req.Parameters["start_time"] = startTime;
+                req.Parameters["start_time"] = startTime.Value.ToString("yyyy-MM-dd");
             }
             if (endTime != null)
             {
-                req.Parameters["end_time"] = endTime;
+                req.Parameters["end_time"] = endTime.Value.ToString("yyyy-MM-dd");
             }
 
             req.OnResponse = OnGetTrainingUsageResponse;