updating adapters to follow new root logic

lib/active_model/serializer/adapter/flatten_json.rb

@@ -1,5 +1,3 @@
-require 'active_model/serializer/adapter/json/fragment_cache'
-
 module ActiveModel
   class Serializer
     class Adapter

lib/active_model/serializer/adapter/json_api.rb

@@ -6,7 +6,6 @@ module ActiveModel
       class JsonApi < Adapter
         def initialize(serializer, options = {})
           super
-          serializer.root = true
           @hash = { data: [] }
 
           if fields = options.delete(:fields)

lib/active_model/serializer/array_serializer.rb

@@ -21,7 +21,7 @@ module ActiveModel
       end
 
       def json_key
-        @objects.first.json_key if @objects.first
+        @objects.first.json_key.pluralize if @objects.first
       end
 
       def root=(root)