Better TimeSpan comparison with zero (#69)

It's a bad practice to compare double values with zero.
A better way to compare TimeSpan with zero is using TimeSpan.Zero.

src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Windows/Markup/DateTimeValueSerializer.cs

@@ -124,7 +124,7 @@ public override string ConvertToString(object value, IValueSerializerContext con
             // Build up the format string to be used in DateTime.ToString()
             StringBuilder formatString = new StringBuilder("yyyy-MM-dd");
             
-            if (dateTime.TimeOfDay.TotalSeconds == 0)
+            if (dateTime.TimeOfDay == TimeSpan.Zero)
             {
                 // The time portion of this DateTime is exactly at midnight.  
                 // We don't include the time component if the Kind is unspecified.