参考: https://github.com/rspec/rspec-rails/issues/1655
header を含むテストは spec/controllers/ ではなく spec/requests/ (type: :request) で書く。
例
require 'rails_helper'
RSpec.describe "Tags", type: :request do
it "returns a success response" do
get "/tags/#{@tag.id}", headers: headers
end
end
rspec-rails (3.7.1)