I was hoping that the API would raise an error if an unknown opt in type is saved, but it just sets it to "Unknown".

I am imagining added some constants like:

```ruby
module DotMailer
  module OptInType
    SINGLE          = :single
    DOUBLE          = :double
    VERIFIED_DOUBLE = :verified_double
  end
end

contact.opt_in_type = DotMailer::OptInType::SINGLE
```

Do you think these need to be validated in the `opt_in_type=` method?