Avoid mutation of @data from external reference

Added a ternany and .dup to the return of 'with_thing_at' to avoid
return a mutable reference that allowed in memory mutation of the
@data struct inside MockRedis

This commit done on the dime of:
Simply Measured
twitter: @SimplyMeasured
web: http://www.simplymeasured.com

Change-Id: I7ea498929a9183f83f40b6f4e6589b7aaacd397b
Reviewed-on: https://gerrit.causes.com/19655
Reviewed-by: Aiden Scandella <aiden@causes.com>
Tested-by: jenkins <jenkins@causes.com>