ActiveModel Serializers support to serialization of a model nested within a hash (overriding default to_json behavior)

Covered the ActiveRecord::SerializerOverride module with a test, and added documentation for when it is useful as I encountered a problem recently with a client that has a lot of legacy code triggering json serialization for a model nested within a hash directly from controller actions, and we could not simply refactor the model to rely on active model serializers without going and changing all the controller actions because the actions were rendering the json of a hash that contained the model within instead of the model directly. 

Here is the documentation describing the problem included with the code change:

## Serialization of a hash containing a nested (embedded) model

There is a mixin that overrides ActiveRecord::Base#to_json and #as_json to enable
serialization via ActiveModel Serializers. We do not recommend that you use ActiveModel
Serializers in this way in general, but there is one exception. That is when a legacy
app's controller actions for example are rendering json for a model nested within a hash
(e.g. {param1 => value1, param2 => model}), which will not trigger ActiveModel Serializers
for the nested model by default, yet instead rely on normal to_json serialization.
To get around this problem, you can require the file 'active_record/serializer_override'
and include the module ActiveRecord::SerializerOverride, which will override the to_json
and as_json methods of the model to rely on ActiveModel Serializers instead.

Example:

In controller actions of a legacy app, there is a lot of this:

```ruby
render :json => {param1 => value1, params2 => value2, data => model}
```

Add an initializer file: "config/initializers/active_model_serializers.rb"

with this content:

```ruby
require 'active_record/serializer_override'
ActiveRecord::Base.send(:include, ActiveRecord::SerializerOverride)
```

or to override to_json and as_json for one model class only:

```ruby
require 'active_record/serializer_override'
ActiveRecord::Base.send(:include, ActiveRecordModel)
```