[Subdoc] Wholedoc mutations do not update datatype to match new document

When Couchbase Server sends a DCP Mutation packet for a binary document with extended attributes the datatype field is incorrectly set to 5 (JSON + XAttrs) and not 4 (Raw + XAttrs).

The earliest version of Couchbase Server I have tested is 5.0.1 (because there isn't a docker container for 5.0.0).

I have included a tcpdump which should contain an effected mutation packet if filtered as 'couchbase and couchbase.opcode == 0x57'

Steps to reproduce:
1, Spin up a Couchbase 5.0.1 docker/vagrant cluster container.
2, Use a later version of cbworkloadgen (which supports xattrs) to insert a single binary document with xattrs.
3, Start tcpdump listening for network from Couchbase Server 'tcpdump -i any -C 1024 -W 10 -w dump.pcap -s 0 port 11210'.
4, Create a backup of the previously created bucket using cbbackupmgr.
5, Look at the datatype for the relevant dcp mutation packet sent by Couchbase Server.