Added unit tests showing that GC'ed SoftReferences are not cleanup from memoize caches
Fixed cache's SoftReferences instantiation in order to register them in a ReferenceQueue
Merge pull request #128 from luque/memoize-cleanupcache-bugfix

[GROOVY-5948]  memoizeAtLeast and memoizeBetween don't cleanup garbage-collected SoftReferences