Follow up to #1535

- The removed classes and modules were added back with deprecation
  warning and deprecation test were added for them.
- One test was renamed because it contained `__`.
- Some tests were refactored.
- The ActiveModelSerializers::Deserialization module is now called
  Adapter instead of ActiveModelSerializers::Adapter.
- The changelog was added for #1535

lib/active_model/serializer/adapter/null.rb

@@ -0,0 +1,14 @@
+require 'active_model_serializers/adapter/null'
+
+module ActiveModel
+  class Serializer
+    module Adapter
+      class Null < DelegateClass(ActiveModelSerializers::Adapter::Null)
+        def initialize(serializer, options = {})
+          warn "Calling deprecated #{self.class.name} (#{__FILE__}) from #{caller[0..2].join(', ')}. Please use #{self.class.name.sub('ActiveModel::Serializer', 'ActiveModelSerializers')}"
+          super(ActiveModelSerializers::Adapter::Null.new(serializer, options))
+        end
+      end
+    end
+  end
+end