Allow a controller to properly override scope_name

Currently the documentation states that you can override the default controller scope and scope name on a per action basis however the scope_name being passed in to override the default is not being used.

This pull request allows you to provide a scope_name and have it carry over into the serializers used.

Example usage taken from README.

```ruby
class CitiesController < ApplicationController
  serialization_scope nil

  def index
    @cities = City.all

    render :json => @cities, :each_serializer => CitySerializer
  end

  def show
    @city = City.find(params[:id])

    render :json => @city, :scope => current_admin, :scope_name => :current_admin
  end
end
```