[FIX] mass_mailing: make campaign default to non-A/B testing

In A/B testing mode, a given recipient will only ever receive a single
email from a given mass-mailing campaign, no matter how many mailings
were sent to them.

This is useful for A/B-testing various mailings in order to check their
results. But is very difficult to understand for users who simply want
to organize their mailings into campaigns.

Additionally, the A/B testing option is hidden as a technical feature, so
it is hard to discover in order to troubleshoot unexpected number of
emails sent by a given mailing.

It will be safer to keep it off by default.
(keeping the parameter explicit, to better show that it is on purpose)

This partially reverts dee5c312

addons/mass_mailing/models/mass_mailing.py

@@ -153,7 +153,7 @@ class MassMailingCampaign(models.Model):
     mass_mailing_ids = fields.One2many(
         'mail.mass_mailing', 'mass_mailing_campaign_id',
         string='Mass Mailings')
-    unique_ab_testing = fields.Boolean(string='Allow A/B Testing', default=True,
+    unique_ab_testing = fields.Boolean(string='Allow A/B Testing', default=False,
         help='If checked, recipients will be mailed only once for the whole campaign. '
              'This lets you send different mailings to randomly selected recipients and test '
              'the effectiveness of the mailings, without causing duplicate messages.')