ci-demand: a NaN cache date keeps a run-list page forever #123

A cache entry whose date is NaN keeps a run-list page forever (it never counts as stale). This only happens when the cache file is damaged by hand.

Fix: treat a non-finite cache date as damaged and fetch the page again; add a test. A failing test was written during the review.

From the quick-check review of #113: #113 (comment)

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