root option is working (fixed #986)

lib/active_model/serializer.rb

@@ -169,7 +169,7 @@ module ActiveModel
     end
 
     def json_key
-      self.class.root_name
+      @root || self.class.root_name
     end
 
     def id

test/serializers/root_test.rb

@@ -0,0 +1,19 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    class RootTest < Minitest::Test
+
+      def setup
+        @profile = Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' })
+        @profile_serializer = ProfileSerializer.new(@post, {root: 'smth'})
+      end
+
+      def test_overwrite_root
+        setup
+        assert_equal('smth', @profile_serializer.json_key)
+      end
+
+    end
+  end
+end