Respect the protocol in `expose_url`

When https is configured in the URL, also use that, and do not set it
to http.

Closes gitlab-org/gitlab-ee#5217

changelogs/unreleased/tc-api-fix-expose_url.yml

+---
+title: Ensure the API returns https links when https is configured
+merge_request: 17681
+author:
+type: fixed

lib/api/helpers/related_resources_helpers.rb

@@ -15,7 +15,7 @@ module API
         url_options = Gitlab::Application.routes.default_url_options
         protocol, host, port = url_options.slice(:protocol, :host, :port).values
 
-        URI::HTTP.build(scheme: protocol, host: host, port: port, path: path).to_s
+        URI::Generic.build(scheme: protocol, host: host, port: port, path: path).to_s
       end
 
       private

spec/lib/api/helpers/related_resources_helpers_spec.rb

+require 'spec_helper'
+
+describe API::Helpers::RelatedResourcesHelpers do
+  subject(:helpers) do
+    Class.new.include(described_class).new
+  end
+
+  describe '#expose_url' do
+    let(:path) { '/api/v4/awesome_endpoint' }
+    subject(:url) { helpers.expose_url(path) }
+
+    def stub_default_url_options(protocol: 'http', host: 'example.com', port: nil)
+      expect(Gitlab::Application.routes).to receive(:default_url_options)
+        .and_return(protocol: protocol, host: host, port: port)
+    end
+
+    it 'respects the protocol if it is HTTP' do
+      stub_default_url_options(protocol: 'http')
+
+      is_expected.to start_with('http://')
+    end
+
+    it 'respects the protocol if it is HTTPS' do
+      stub_default_url_options(protocol: 'https')
+
+      is_expected.to start_with('https://')
+    end
+
+    it 'accepts port to be nil' do
+      stub_default_url_options(port: nil)
+
+      is_expected.to start_with('http://example.com/')
+    end
+
+    it 'includes port if provided' do
+      stub_default_url_options(port: 8080)
+
+      is_expected.to start_with('http://example.com:8080/')
+    end
+  end
+end