Preview does not work with max-age cache

[angry-dan]

Hello, I've noticed that previews don't work when used in combination with caching.

Here's an example:

I've got a Node Express server which listens to two endpoints:

example.com/preview - which interprets prismic preview requests
example.com/* - which reads a uid and returns a page.

The second of those two endpoints also emits a cache-control header allowing the browser (and our CDN) to cache the page for 2 hours.

When you activate the preview in prismic the following happens:

• browser requests example.com/preview
• express server emits a 301 redirect to example.com/some-page
• express server serves live version of example.come/some-page
• prismic toolbar phones home, gets a preview ref tries to reload the page
• expected result: browser reloads the page and user sees draft version.
  actual result: browser reloads the page from it's local cache not from the server, user still sees the live version.

This happens because the method of refreshing here - https://github.com/prismicio/prismic-toolbar/blob/master/src/share.js#L29 - is to just set the window location to effectively be window.location. Really at this point it should call window.location.reload(true).

The CDN isn't an issue here, because it's already varying responses based on the cookie that gets set. It's the browser not making the request that's the problem.