support different serializations in ArraySerializer

ArraySerializer can serialize arrays of Hash, ActiveModel::Serializer,
or any PORO that responds to :as_json

Each item in the array should not include its type as a root element,
since normal arrays do not do this. If you want "typed" collections,
don't use an Array.

lib/active_model/serializer.rb

@@ -52,7 +52,15 @@ module ActiveModel
       @options[:hash] = hash = {}
       @options[:unique_values] = {}
 
-      array = serializable_array.map(&:serializable_hash)
+      array = serializable_array.map do |item|
+        if item.is_a?(Hash)
+          item
+        elsif item.respond_to?(:serializable_hash)
+          item.serializable_hash
+        else
+          item.as_json
+        end
+      end
 
       if root = @options[:root]
         hash.merge!(root => array)