assign objects in a way to keep identity

[javierg]

Object identity for hash is lost when we switch from Hash to HashWithIndifferentAccess for role property values.

Current behaivor:

class Model < Dolly::Document
  property :foo, class_name: Hash
end

h = {}
doc = Model.new(foo: h)

doc.foo.object_id == doc.send(:doc)[:foo].object_id # => false

Expects:

doc.foo.object_id == doc.send(:doc)[:foo].object_id # => true