fix checking of mailer services in EmailSenderListener
merged branch smatyas/emailsenderlistener-fix (PR #44)

This PR was squashed before being merged into the master branch (closes #44).

Discussion
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fix checking of mailer services in EmailSenderListener

The 'initialized' method of the Symfony\Component\DependencyInjection\Container does not handle aliases (as it is in the ContainerBuilder), but therefore the EmailSenderListener never sends emails.

This fix is related to issue #43.

Commits
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33a7015 fix checking of mailer services in EmailSenderListener