Handle case where key has expired or been deleted

[dustinmartin]

This PR fixes a bug caused by express-redis-cache attempting to get a key that has been deleted or expired.

This scenarios happens like this:

1. key is looked up in Redis get.js, line 36
2. key expires or is deleted from Redis
3. key is retrieved from Redis get.js, line 43. hgetall returns null since the key no longer exists.
4. the function attempts to set properties on the null value, get.js, line 45 which results in an error being thrown and the app crashing.

I ran the tests and they all pass though I did not add a new test. Due to the nature of this bug I wasn't sure how to go about creating a new test. Please let me know if I need to make some changes. Thanks.

[krazyjakee]

#84