feat(mailer): disable X-Mailer header in emails (#1881) r=vladikoff,philbooth

lib/senders/email.js

@@ -231,6 +231,7 @@ module.exports = function (log) {
       subject: localized.subject,
       text: localized.text,
       html: localized.html,
+      xMailer: false,
       headers: extend({
         'Content-Language': localized.language,
         'X-Template-Name': message.template

test/remote/account_create_tests.js

@@ -81,6 +81,7 @@ describe('remote account create', function() {
         )
         .then(
           function (emailData) {
+            assert.equal(emailData.headers['x-mailer'], undefined)
             assert.equal(emailData.headers['x-template-name'], 'verifySyncEmail')
             assert.equal(emailData.html.indexOf('IP address') > -1, true) // Ensure some location data is present
             return emailData.headers['x-verify-code']