Added a trinay 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

Spec is probably not consistent with present spec structure but
does illustrate a red/green against method. Spec might also need
be moved as it effects more then just hgetall.

This commit done on the dime of:
Simply Measured
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