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.

test/fixtures/poro.rb

@@ -26,6 +26,8 @@ class Profile < Model
 end
 
 class Post < Model
+  attr_writer :comments
+
   def comments
     @comments ||= [Comment.new(content: 'C1'),
                    Comment.new(content: 'C2')]