Fix Cash::Mock#get with multiple keys / #set without a ttl

Fix Cash::Mock#get with multiple keys / #set without a ttl Edit I added the ability to test the Cash::Mock implementation via the ADAPTER hook. This revealed a few issues with the implementation. Specifically:

When #get is invoked on Cash::Mock with an array of keys, it fails to do a multi-get
When #set is invoked with a nil ttl, Cash::Mock::CacheEntry fails to fall back to the default

This changeset also fixes those issues.

ngmoco / cache-money

Fix Cash::Mock#get with multiple keys / #set without a ttl #23