History traversal fetches should be "force-cache", or "only-if-cached"

[jakearchibald]

From poking browsers, it seems like, when you traverse to a history entry without a document, that document is fetched using cache mode:

In Firefox/Chrome:

• "force-cache" if the request is GET.
• "only-if-cached" if the request is POST.

In Safari:

• "force-cache" if the request is GET.
• "default" (maybe?) if the request is POST.

Some of this split is because Safari shows a prompt to resubmit form data on traversal, whereas Firefox/Chrome show an error on traversal. I prefer the Firefox/Chrome behaviour for the reasons in #6600