Improve adapter test coverage per groyoh

lib/active_model/serializer.rb

@@ -23,8 +23,6 @@ module ActiveModel
     include Links
     include Meta
     include Type
-    # Deprecated
-    require 'active_model_serializers/adapter'
 
     # @param resource [ActiveRecord::Base, ActiveModelSerializers::Model]
     # @return [ActiveModel::Serializer]

lib/active_model_serializers.rb

@@ -9,6 +9,7 @@ module ActiveModelSerializers
   autoload :Deserialization
   autoload :Logging
   autoload :Test
+  autoload :Adapter
 
   class << self; attr_accessor :logger; end
   self.logger = ActiveSupport::TaggedLogging.new(ActiveSupport::Logger.new(STDOUT))