ben-manes / caffeine / #5314

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fix putIfAbsent calculating the read expiry from the offered new value

When the value is present in the cache the offered new value for insert
is ignored and the current value is returned. If using expireAfter then
Expiry's expireAfterRead(..) is called to calculate the new expiration
time if changed. This was being passed the abandoned new value, not
the existing value, which was incorrect. The unit test for this case
was accidentally verifying the old value was passed in and asserting
this typo.

This call mistake is very rare and should be benign. Typically an
optimistic fast path suceeds to avoid unnecessary locking which
correctly uses the current value. It was only if a race of the entry
appearing expired or GC'd, but a concurrent insert rectified that,
did the lock fallback's incorrect call get used. Most putIfAbsent calls
are for the same value or a memoizing proxy, and then unlikely to
use access expiration or inspect the value for the calculation. That
made it very rare to be negatively impacted by this mistake.

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