Set SwiftMailer creation message as lazy.

At this moment, with a config:

```
swift:
            type:       swift_mailer
            from_email: user@domain.tld
            to_email:   user@domain.tld
            level:      debug
```

A `Swift_Message` instance is created on each places where `logger` is injected. Everybody know the cost of a  `Swift_Message` instance ...