added email custom headers in user verify

I some cases is necesary to add custom header in user.verify, for example to track an email
@bajtos I've simplified the test

I use the result.email.messageId param because the nodemailer-stub-transport(
https://github.com/andris9/nodemailer-stub-transport/blob/master/src/stub-transport.js#L52) return that value to the callback

@ritch