JCBC-245: Allow Object instead of String as value.

This changeset aligns the allowed value types for mutate methods
with persistence constraints to those without constraints.

Change-Id: I529a868c5b88b4b5ea193ca818e972c2367affd6
Reviewed-on: http://review.couchbase.org/24689
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>