ActionMailer: mark mail as called after instead of before processing it

#16163 introduced raising an exception when mail attachments get added after the method `mail` was called since it seems to produce problems with the email body. The change checks if `@_mail_was_called` is true which gets set on one of the first lines in `mail`. I suggest that we move it to the end of the method.

The background why I request this change is that we add inline attachments as the mail view gets rendered by overwriting the `image_tag` view helper. 

```
  def image_tag(path, options={})
    if URI.parse(path).relative?
      controller.attachments.inline[path] ||= File.read("#{Rails.root}/app/assets/images/#{path}")
      path = attachments[path].url
    end
    super(path, options)
  end
```

This way all the images we use get attached transparently. Very convenient.

The above change raises because `@_mail_was_called` the time the view is rendered. The big question is, if my change would reintroduce the problem or support both requirements. Maybe @joakimk or @senny can help with that?