Fixed issue with ActiveRecord serialize object as JSON

With 4.1.x `serialize` started returning a string when `JSON` was passed as the second attribute. It will now return a hash as per previous versions (prior to 4.1.x)

Example: An ActiveRecord model named `Post` with a `comment` column that uses the serialize method to convert a `Comment` object to `JSON`.

```ruby
class Post < ActiveRecord::Base
serialize :comment, JSON
end

class Comment
  include ActiveModel::Model
  attr_accessor :category, :text
end

post = Post.create!
post.comment = Comment.new(category: "Animals", text: "This is a comment about squirrels.")
post.save!

# This will now return a hash
post.comment
```
    
When `JSON` is used with `serialize`, ActiveRecord will use the new `ActiveRecord::Coders::JSON` coder which delegates `ActiveRecord::Coders::JSON.dump/load` to `ActiveSupport::JSON.encode/decode`. 

Fixes #15594
"Restore 4.0 behavior for..." is possibly a more reassuring opening? Just given that this is specifically a behavioural change in a stable release.

I'm loath to suggest making this entry much longer, but should we offer people a solution if they need to retain the previous 4.1 behaviour? Do we have any precedent for a link to a gist or something?