This PR adds Rubocop and code review adjustments on top of #1425:

Quoting @mswiszcz:

Allows to create tests checking set foreign_type

class Visitor < ActiveRecord::Base
  belongs_to :location, foreign_type: :facility_type, polymorphic: true
end

class Hotel < Location
  has_many :visitors, inverse_of: :location, foreign_type: :facility_type, as: :location
end

class PrisonCell < Location
  has_one :visitor, inverse_of: :location, foreign_type: :facility_type, as: :location
end

expect(Visitor.new).to belong_to(:location).with_foreign_type(:facility_type)
expect(Hotel.new).to have_many(:visitors).with_foreign_type(:facility_type)
expect(PrisonCell.new).to have_one(:visitor).with_foreign_type(:facility_type)

All credits go to @mswiszcz for this contribution.