Avoid Mutation of @data from external reference

[kungfumike]

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
twitter: @SimplyMeasured
web: http://www.simplymeasured.com

[kungfumike]

I can move that stuff around, sure.

Trinay is what happens when you try to type 'ternary' high on cold medicine.

I had to further correct this logic to only apply to hashes, arrays, and strings. As certian methods like sadd return 'true' which like nil also can't be duped.

I will get those specs moved around

[kungfumike]

@sectioneight Moved the spec around for you, go ahead and give it another look when you have a chance. Thanks.

[ascandella]

@kungfumike Thanks! We're a little, uh... obsessive about our code style at @causes. If you don't mind, I'll squash these all into one commit, since we don't want any single point in history to have failing specs.

[ascandella]

Cherry-pick to master at a08edf7 courtesy of our Gerrit code review instance. Thanks for fixing this up!