When using embed: :ids ; embed_in_root: true, and serializing multiple objects,

only the associated objects of the last object in the collection will actually
show up in the serialized data.

For example, if you serialize a collection of two posts, each containing one or
more comments, only the comments of the last post show up. The reason is a
Hash#merge wich overwrites the array rather than appending to it.

This commit fixes this by merging the collection arrays, rather than the top-level
hashes.

lib/active_model/array_serializer.rb

@@ -44,7 +44,10 @@ module ActiveModel
 
     def embedded_in_root_associations
       @object.each_with_object({}) do |item, hash|
-        hash.merge!(serializer_for(item).embedded_in_root_associations)
+        serializer_for(item).embedded_in_root_associations.each_pair do |type, objects|
+          hash[type] = hash.fetch(type, []).concat(objects)
+          hash[type].uniq!
+        end
       end
     end
   end