thawing a freezed Column returns a Map

[Daniel Slutsky]

Serializing/Desirializing Columns with Nippy generates surprising results:

$ clj -Sdeps '{:deps {techascent/tech.ml.dataset {:mvn/version "6.085"}}}'
Clojure 1.10.3

user=> (require '[taoensso.nippy :as nippy]
         '[tech.v3.dataset]
         '[tech.v3.dataset.column :as column])

nil

user=> (-> (range 4)
    column/new-column
    nippy/freeze
    nippy/thaw)

#:tech.v3.dataset{:name nil, :missing {}, :force-datatype? true, :data #array-buffer<int64>[4]
[0, 1, 2, 3]}

user=> (-> (range 4)
    column/new-column
    nippy/freeze
    nippy/thaw
    type)

clojure.lang.PersistentArrayMap