Fix NoMethodError for ActivityPub

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/424371

Handle the case when Accept header is missing.

Changelog: fixed

lib/constraints/activity_pub_constrainer.rb

@@ -3,7 +3,7 @@
 module Constraints
   class ActivityPubConstrainer
     def matches?(request)
-      mime_types.any? { |m| request.headers['Accept'].include?(m) }
+      mime_types.any? { |m| request.headers.fetch('Accept', '').include?(m) }
     end
     private

spec/lib/constraints/activity_pub_constrainer_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Constraints::ActivityPubConstrainer, feature_category: :groups_and_projects do
+  subject(:constraint) { described_class.new }
+
+  describe '#matches?' do
+    subject { constraint.matches?(request) }
+
+    let(:request) { ActionDispatch::Request.new(headers) }
+
+    ['application/ld+json; profile="https://www.w3.org/ns/activitystreams"', 'application/activity+json'].each do |mime|
+      context "when mime is #{mime}" do
+        let(:headers) { { 'HTTP_ACCEPT' => mime } }
+
+        it 'matches the header' do
+          is_expected.to be_truthy
+        end
+      end
+    end
+
+    context 'when Accept header is missing' do
+      let(:headers) { {} }
+
+      it 'does not match' do
+        is_expected.to be_falsey
+      end
+    end
+  end
+end