Make Associations in root work with ArraySerializer

Closes #414

lib/active_model/array_serializer.rb

@@ -37,5 +37,27 @@ module ActiveModel
       end
     end
     alias_method :serializable_object, :serializable_array
+
+    def serializable_data
+      embedded_in_root_associations.merge!(super)
+    end
+
+    def embedded_in_root_associations
+      hash = {}
+      @object.map do |item|
+        serializer_class = @each_serializer || Serializer.serializer_for(item) || DefaultSerializer
+        associations = serializer_class._associations
+        serializer = serializer_class.new(item, @options)
+        included_associations = serializer.filter(associations.keys)
+        associations.each do |(name, association)|
+          if included_associations.include? name
+            if association.embed_in_root?
+              hash[association.embedded_key] = serializer.serialize association
+            end
+          end
+        end
+      end
+      hash
+    end
   end
 end