Update SimpleText.cs

Fixed Issues : #391
修复SimpleText在更改Foreground的时候不会应用更改的问题

src/Shared/HandyControl_Shared/Controls/Text/SimpleText.cs

@@ -170,6 +170,7 @@ namespace HandyControl.Controls
             _formattedText.SetFontWeight(FontWeight);
             _formattedText.SetFontFamily(FontFamily);
             _formattedText.SetFontStretch(FontStretch);
+            _formattedText.SetForegroundBrush(Foreground);
         }
 
         private static void OnFormattedTextUpdated(DependencyObject d, DependencyPropertyChangedEventArgs e)