MB-100 return a reference for operator =

For correctness and avoiding unnessary memory copy
overloaded *() operator must return reference instead
of referencing object itself.

Newer versions of gcc (like 4.8) would fails compiling
without the fix.

Change-Id: Id5c747c2a17cff57c6d7d20f567509375677ef32
Reviewed-on: http://review.couchbase.org/26230
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Jin Lim <jin@couchbase.com>