Whitelist specific redirect URLs as an alternative to Vaadin-Refresh

Based on a discussion in https://stackoverflow.com/a/60389722/2376954, there may be situations where it’s not practical to add the Vaadin-Refresh token to a redirected or otherwise intercepted response.

As an alternative, the client engine could detect whether a redirect has happened (either by migrating to the fetch API or by looking at whether responseURL has changed) and if the new URL matches a rule (e.g. a regular expression) configured for the application, then the new target URL is loaded as a top-level navigation instead of interpreting the response as UIDL.

[Legioth]

It may actually even make sense to look at the content type of the response as well. If it’s e.g. text/html instead of the expected application/json, then it may be quite safe to assume that a top-level navigation to the same URL may be appropriate.

[Legioth]

I guess the main reason for not sending an Accept header is that nobody has realized it would make sense to send one. One such reason has now been discovered.