Fix ArraySerializer's output when falling back on DefaultSerializer

Change ArraySerializer's initialization of DefaultSerializer to pass
:root => false in order to avoid adding root keys to each of the
items in the array. This ensures consistent results between serializing
arrays of objects with serializers and arrays of objects without
serializers. Fixes #495.

lib/active_model/array_serializer.rb

@@ -83,7 +83,7 @@ module ActiveModel
           serializer = item.active_model_serializer
         end
 
-        serializable = serializer ? serializer.new(item, @options) : DefaultSerializer.new(item, @options)
+        serializable = serializer ? serializer.new(item, @options) : DefaultSerializer.new(item, @options.merge(:root => false))
 
         if serializable.respond_to?(:serializable_hash)
           serializable.serializable_hash

test/array_serializer_test.rb

@@ -2,6 +2,27 @@ require "test_helper"
 require "test_fakes"
 
 class ArraySerializerTest < ActiveModel::TestCase
+
+  def test_array_items_do_not_have_root
+    array = [
+        BasicActiveModel.new(:name => "First model"),
+        BasicActiveModel.new(:name => "Second model")
+    ]
+    expected = { "root" => [
+        { :name => "First model" },
+        { :name => "Second model" }
+    ] }
+
+    default_serializer = array.active_model_serializer.new(array, :root => "root")
+    each_serializer = array.active_model_serializer.new(array, :root => "root", :each_serializer => BasicSerializer)
+
+    default_json = default_serializer.as_json
+    each_json = each_serializer.as_json
+
+    assert_equal(expected, default_json)
+    assert_equal(expected, each_json)
+  end
+
   # serialize different typed objects
   def test_array_serializer
     model    = Model.new

test/test_fakes.rb

@@ -1,3 +1,26 @@
+class BasicActiveModel
+  include ActiveModel::Serializers::JSON
+  def initialize(hash = {})
+    @attributes = hash
+  end
+
+  def serializable_hash(*)
+    @attributes
+  end
+
+  def method_missing(method)
+    if @attributes.key? method
+      @attributes[method]
+    else
+      raise NoMethodError
+    end
+  end
+end
+
+class BasicSerializer < ActiveModel::Serializer
+  attributes :name
+end
+
 class Model
   def initialize(hash={})
     @attributes = hash