Accordion: expression is mapped in panel-body, but not in panel-heading

[stevenbirr]

Hello,

I found a bug in the accordion component. I've updated my accordion example: https://github.com/stevenbirr/angular-ui.dart.accordion-example

As you can see, the model variable valueInPanelBody is correctly mapped into the panel body. Unfortunately, the variable valueInPanelHeading is not correctly mapped into the panel heading, see the image.

Maybe, this is related to #153