remove serializer dependency from fieldset

test/adapter/json_api/fieldset_test.rb

@@ -17,7 +17,7 @@ module ActiveModel
           end
 
           def test_fieldset_with_fields_array
-            fieldset = ActiveModel::Serializer::Fieldset.new(@serializer, ['title'])
+            fieldset = ActiveModel::Serializer::Fieldset.new(['title'], 'post')
 
             assert_equal(
               {:title=>"New Post", :links=>{:comments=>["1", "2"]}}, 
@@ -26,7 +26,7 @@ module ActiveModel
           end
 
           def test_fieldset_with_hash
-            fieldset = ActiveModel::Serializer::Fieldset.new(@serializer, {post: [:body]})
+            fieldset = ActiveModel::Serializer::Fieldset.new({post: [:body]})
 
             assert_equal(
               {:body=>"Body", :links=>{:comments=>["1", "2"]}}, 
@@ -35,7 +35,7 @@ module ActiveModel
           end
 
           def test_fieldset_with_multiple_hashes
-            fieldset = ActiveModel::Serializer::Fieldset.new(@serializer, {post: [:title], comment: [:body]})
+            fieldset = ActiveModel::Serializer::Fieldset.new({post: [:title], comment: [:body]})
 
             assert_equal(
               [{:body=>"comment one" }, {:body=>"comment two"}],

test/serializers/fieldset_test.rb

@@ -0,0 +1,26 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    class FieldsetTest < Minitest::Test
+
+      def test_fieldset_with_hash
+        fieldset = ActiveModel::Serializer::Fieldset.new({'post' => ['id', 'title'], 'coment' => ['body']})
+
+        assert_equal(
+          {:post=>[:id, :title], :coment=>[:body]}, 
+          fieldset.fields
+        )
+      end
+
+      def test_fieldset_with_array_of_fields_and_root_name
+        fieldset = ActiveModel::Serializer::Fieldset.new(['title'], 'post')
+
+        assert_equal(
+          {:post => [:title]}, 
+          fieldset.fields
+        )
+      end
+    end
+  end
+end