[isomorphic-core] Disable nodemailer's automatic X-Mailer header
[isomorphic-core] Disable nodemailer's automatic X-Mailer header

Examined the headers on a message we sent and found this:
> X-Mailer: nodemailer (2.5.0; +http://nodemailer.com/; SMTP/2.6.0[client:2.8.0])

No need to plaster which sendmail library we're using all over every
email our users send. Turn it off!

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