Adding block override for preloading with has_one.

[jwoertink]

Fixes #985

The has_many and belongs_to both have a preload override method that takes a block and passes in the BaseQuery object. The has_one was missing it.

This lets you do something like

class User < BaseModel
  table do
    has_one theme : Theme
  end
end

UserQuery.new.preload_theme(&.with_soft_deleted)