UnsupportedBrowserHandler doesn't include Content-Type-Header in response

[StefanPenndorf]

The error page generated by UnsupportedBrowserHandler doesn't add a Content-Type header. Thus chrome offers the page for download instead of displaying the error message.

Reproduce:

• Configure chrome developer tools to use an ancient browser (e.g. IE 7)
• visit bakery app starter: https://bakery-flow.demo.vaadin.com/login
• See a download of "login" file starting

Can be fixed by adding

        response.setContentType("text/html charset=utf-8");

Affected Versions: all (even UnsupportedBrowserHandler in Vaadin 8 seems affected although I found the bug evaluating a Vaadin 14 application).