Always show the association key even if there is no data

[khiav223577]

Something if we deep_pluck a model with assocation, Ex:

User.deep_pluck(:name, 'city' => :name)

The result hash may not contain 'city' key in that the user doesn't have a city. Ex:

{ 'name' => 'Pearl' }

We want the result always contain the key, Ex:

{ 'name' => 'Pearl', 'city' => nil }