turbo:frame-missing: Re-use FetchResponse HTML

[seanpdoyle]

After re-considering a comment on hotwired/turbo#445, this commit
re-purposes the response HTML from the FetchResponse instance, instead
of issuing a follow-up HTTP request.

[dhh]

Nice. This will save that extra request 👍

[willcosgrove]

Doesn't the response not include a layout? I know there was talk of always including the layout for frame requests, but it sounds like that was dismissed (hotwired/turbo-rails#232 (comment)).

I'm experiencing an issue where I have a form contained within a Turbo Frame, and on submit I want to redirect to a different page that does not contain a matching frame. What happens is:

1. The frame handles the form submit
2. It gets a redirect response
3. It follows the redirect (still including the Turbo-Frame header)
4. The document fetched as a result of the redirect has no matching frame, and no layout
5. This response document replaces the body because it is being promoted to a full page visit. But because it has no layout, it breaks the page.

It seems like the two options to fix this are either to 1. remove the layout: false optimization for frame requests, or 2. issue two requests in this specific scenario. The first one follows the redirect with the Turbo-Frame header, sees it has no matching frame, and then performs a visit to the same URL but this time with no Turbo-Frame header so that the full layout is included.