remove serializer dependency from fieldset

lib/active_model/serializer/fieldset.rb

@@ -2,26 +2,29 @@ module ActiveModel
   class Serializer
     class Fieldset
 
-      attr_accessor :fields, :root
+      attr_reader :fields, :root
 
-      def initialize(serializer, fields = {})
-        @root   = serializer.json_key
+      def initialize(fields, root = nil)
+        @root   = root
         @fields = parse(fields)
       end
 
       def fields_for(serializer)
         key = serializer.json_key || serializer.class.root_name
-        fields[key]
+        fields[key.to_sym]
       end
 
     private
 
       def parse(fields)
         if fields.is_a?(Hash)
-          fields.inject({}) { |h,(k,v)| h[k.to_s] = v.map(&:to_sym); h}
+          fields.inject({}) { |h,(k,v)| h[k.to_sym] = v.map(&:to_sym); h}
         elsif fields.is_a?(Array)
+          if root.nil?
+            raise ArgumentError, 'The root argument must be specified if the fileds argument is an array.'
+          end
           hash = {}
-          hash[root.to_s] = fields.map(&:to_sym)
+          hash[root.to_sym] = fields.map(&:to_sym)
           hash
         else
           {}