propagate datatype field to #upr_mutation{}

Change-Id: I7ffefaa3f7e3008ebefaf5e3756ae50fdc4918f8
Reviewed-on: http://review.couchbase.org/37554
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>