Using old Postbank account numbers as integer the validation should succeed

Hi,

After updating to version 0.2.0 the validation resulted in the library throwing errors:

```ruby
NoMethodError:
       undefined method `length' for 123456789123456:Fixnum
```

My application used old Postbank account numbers that are saved as integers. I updated the `validate_account_number` method for non-string input and added some tests for this use-case.