Always set a serializer for each item of an Array

model.active_model_serializer could return nil so we need to ensure that
if serializer is not setted we set DefaultSerializer to it.

This reverts commit 64ed05c484.

Fixes #318

lib/active_model/array_serializer.rb

@@ -49,9 +49,8 @@ module ActiveModel
           serializer = options[:each_serializer]
         elsif item.respond_to?(:active_model_serializer)
           serializer = item.active_model_serializer
-        else
-          serializer = DefaultSerializer
         end
+        serializer ||= DefaultSerializer
 
         serializable = serializer.new(item, options.merge(:root => nil))
 

test/array_serializer_test.rb

@@ -52,7 +52,7 @@ class ArraySerializerTest < ActiveModel::TestCase
     assert_equal({ :items => [ hash.as_json ]}, serializer.as_json)
   end
 
-  def test_array_serializer_with_specified_seriailizer
+  def test_array_serializer_with_specified_serializer
     post1 = Post.new(:title => "Post1", :author => "Author1", :id => 1)
     post2 = Post.new(:title => "Post2", :author => "Author2", :id => 2)
 
@@ -65,4 +65,21 @@ class ArraySerializerTest < ActiveModel::TestCase
       { :title => "Post2" }
     ], serializer.as_json)
   end
+
+  def test_array_serializer_using_default_serializer
+    hash = { "value" => "something" }
+    class << hash
+      def active_model_serializer
+        nil
+      end
+    end
+
+    array = [hash]
+
+    serializer = array.active_model_serializer.new array
+
+    assert_equal([
+      { "value" => "something" }
+    ], serializer.as_json)
+  end
 end