Sending emails in a loop causing _formData to persist

[ajibarra]

In Cake4, Mailer class clone transport before executing deliver method, then it restores the transport instance. When MailgunTransport is clone, the FormData instance in _formData property is shared between the two instances (original and clone) so even when _reset is called after sending email, the cloned instance keeps the previous _formData generating emails with multiple to, subject and from headers.

Calling _reset in new instance fixes the issue