feature: better attributes inspect.

Datamappify class inspection:
```ruby
#<User id: Integer, first_name: String, last_name: String, age: Integer, passport: String, driver_license: String, health_care: String>
```
Datamappify instance inspection:
```ruby
#<User id: nil, first_name: nil, last_name: nil, age: nil, passport: nil, driver_license: nil, health_care: nil>
```