Correctly write stroke if it has the default value

- if the element would have another inherited stroke value,
  the "none" value was not written anyway
- fixes writing of painting-marker-01-f.svg and painting-marker-06-f.svg

Source/SvgElement.cs

@@ -585,7 +585,7 @@ namespace Svg
                     string value = (string)attr.Property.Converter.ConvertTo(propertyValue, typeof(string));
 
                     forceWrite = false;
-                    writeStyle = (attr.Attribute.Name == "fill");
+                    writeStyle = attr.Attribute.Name == "fill" || attr.Attribute.Name == "stroke";
 
                     if (writeStyle && (Parent != null))
                     {