Fixup datatype when we receive tap mutation

Change-Id: Icd5c333b3939130ad8a51dcfbeae33b83344fe55
Reviewed-on: http://review.couchbase.org/36750
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>