Fix bug related to riak_dt#110 serialization changes

Previously, Mod:from_binary/1 would just return the Datatype, but now it
returns {ok, Datatype}. Fixed this in the one place it was called here
and added a quickcheck property for it.
Merge pull request #1065 from basho/bugfix/sdc/crdt-from-binary-tagged-ok

Fix bug related to riak_dt#110 serialization changes

Reviewed-by: russelldb