Use 'when logging as guest' for context, feedback:

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5347#note_13200997

spec/requests/api/builds_spec.rb

@@ -194,7 +194,7 @@ describe API::API, api: true do
       end
     end
 
-    context 'when forbidden' do
+    context 'when logging as guest' do
       let(:api_user) { guest_user }
 
       before do