Allow resetting singular associations

[georgeclaghorn]

has_one :author and belongs_to :author each define a reload_author method on the owning model. reload_author clears the cached author associate-object and eagerly fetches a new one from the database.

In some cases, you know the cached associate is stale but not whether it will be needed again. If it isn’t, the database query from calling reload_* is wasted.

Consider the following models:

class Account < ApplicationRecord
  has_many :trials
  has_one :active_trial, -> { active }, class_name: "Trial"
end

class Trial < ApplicationRecord
  belongs_to :account
  
  scope :active, -> { where active: true }

  def deactivate
    update! active: false
    # 📍
  end
end

At the 📍 in Trial#deactivate, we know we’ve potentially made account’s active_trial association stale. We don’t know if account will need to reference its active_trial again. It would be useful to be able to evict the cached active_trial, if any, and defer the database query until the next access.

For collection associations, ActiveRecord::Associations::CollectionProxy provides a #reset method in addition to #reload. There’s not an equivalent way to reset a singular association, though. This PR adds one.