Re-add login_spec.rb after fixing CI tests

This reverts commit a7ebf1d which removed login_spec.rb added in earlier
commits. I have simply re-added the login_spec with a couple of lines of
comments hinting at the TypeError which caused its temporary removal and
a link with more information (should this hit anyone in the future)
Merge pull request #101 from gds-operations/login-spec-test

Re-add login_spec.rb after fixing CI tests