CCACHE-28: fixes concurrency bug in has?

We were allowing for another thread to add an item to the cache after we'd
already tried to pull out the SoftReference.  Since the SoftReference had been
pulled out before the item was added, it was nil, but since contains? returned
true, we assumed that it was not nil.

Signed-off-by: Ambrose Bonnaire-Sergeant <abonnairesergeant@gmail.com>
Update change log with CCACHE-28