Deprecate calling "preference" without including Preferable

The error message here calls for "persistable", because all objects that
inherit from Spree::Base *actually* need the Persistable module.

core/app/models/spree/base.rb

@@ -6,6 +6,22 @@ class Spree::Base < ActiveRecord::Base
   include Spree::Core::Permalinks
   include Spree::RansackableAttributes
 
+  def self.preference(*args)
+    Spree::Deprecation.warn <<~WARN
+      #{name} has a `preferences` column, but does not explicitly (de)serialize this column.
+      In order to make #{name} work with future versions of Solidus (and Rails), please add the
+      following line to your class:
+      ```
+      class #{name}
+        include Spree::Preferences::Persistable
+        ...
+      end
+      ```
+    WARN
+    include Spree::Preferences::Persistable
+    preference(*args)
+  end
+
   def preferences
     value = read_attribute(:preferences)
     if !value.is_a?(Hash)

core/spec/models/spree/base_spec.rb

@@ -45,13 +45,18 @@ class PrefTestWithoutSerialization < Spree::Base
 
     it "returns a Hash nevertheless" do
       instance = PrefTestWithoutSerialization.new
-      expect(instance.preferences).to eq({})
+      expect(instance.preferences).to be_a(Hash)
     end
 
     it "returns a Hash when there's already values in the table" do
       ActiveRecord::Base.connection.execute("INSERT INTO pref_tests (col, preferences) VALUES ('test', '---\n:percent: 20')")
       instance = PrefTestWithoutSerialization.first
-      expect(instance.preferences).to eq(percent: 20)
+      expect(instance.preferences).to include(percent: 20)
+    end
+
+    it "includes the persistable module when calling #preference and sets the preference default" do
+      PrefTestWithoutSerialization.preference :percentage, :number, default: 5
+      expect(PrefTestWithoutSerialization.new.preferences).to eq(percentage: 5)
     end
   end
 end