I've created a scenario in the spec file for the case when the configuration of the adapter does not contain a hash for `extra_attributes`.

When the user sets the `extra_attributes` 
```ruby
{ 
  email: 'mail_address', 
  other: 'field' 
}.to_a # => [[:email, 'mail_address'], [:other, 'field']]
```
when iteration through on this nested array, the inner array will assigned to the two variables between the pipes with decompositional assignment.

When the user does not give a mapping, the option is nil. Before this type cast, it raised a NoMethodError because of the `each` call on `nil`.

All in all, from now `extra_attributes` is optional.