remove unncessary is_a?(Hash) check

lib/active_model/serializer.rb

@@ -53,9 +53,7 @@ module ActiveModel
       @options[:unique_values] = {}
 
       array = serializable_array.map do |item|
-        if item.is_a?(Hash)
-          item
-        elsif item.respond_to?(:serializable_hash)
+        if item.respond_to?(:serializable_hash)
           item.serializable_hash
         else
           item.as_json

test/serializer_test.rb

@@ -408,7 +408,7 @@ class SerializerTest < ActiveModel::TestCase
     hash = {:value => "something"}
     array = [hash]
     serializer = array.active_model_serializer.new(array, :root => :items)
-    assert_equal({ :items => [ hash ]}, serializer.as_json)
+    assert_equal({ :items => [ hash.as_json ]}, serializer.as_json)
   end
 
   class CustomBlog < Blog