Missing style scope for header in the Kitchen Sink test page

[platosha]

Description

When using the Kitchen Sink page in Material Theme repo on Firefox / Edge / IE, the header cells are missing the style-scope vaadin-grid classes. At the same time, the style scope is applied to body cells.

Expected outcome

The header looks the same as in Chrome.

Actual outcome

The visual appearance of the grid header is different, the theme is not getting applied to the header cells.

Steps to reproduce

1. Run polymer serve in the Material Theme repo: https://github.com/vaadin/vaadin-material-theme
2. Open test/sink.html in Chrome and Firefox
3. Compare the look.

Browsers Affected

• Chrome
• Firefox
• Safari
• Edge
• IE 11
• iOS Safari
• Android Chrome

[jouni]

I believe this is a duplicate of #1227