Fix MailerPreview broken tests

`BaseMailerPreview.welcome` is an instance method, so we need to stub the
method on a instance level and not on Class. The stub is important to make
sure the Message object is the same in the other expectations.
This was working randomly because Mocha uses == to compare two objects
on the `with()` expectation and even if the Mail::Message objects were
not the same object they are equal, but thats not the case in 100% of
the runs. So we need to make sure we use `.any_instance` method and have
the right message object.

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