fix failing "distribute_to_commits.feature" scenario

* Scenario: Distribute to commits not linked to a GitHub account

User#recipient_label method was prefering to show identifier instead of email when nick was undefined but the test was expecting an email address - not sure which was the intended behavior but showing email to the user rather than an obtuse ID seems more reasonable

is it possible this could be a private email address ? then perhaps to modify the test instead - the original implementation would still show it however in cases where no nick nor ID is defined if such a state is possible