MB-100 Don't update rep document when nothing has changed.

Change-Id: I7389eb00e346afb668fbfe460e6eaeea40fe48fb
Reviewed-on: http://review.couchbase.org/28368
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>