Delegate #to_model method to model

`ActiveModel::Conversion#to_model` is required to build correct URLs with form object.

Before patch:

```ruby
form_object = ApplicantForm.new(applicant: applicant, profile: applicant.profile)
url_for(form_object) #=> /applicants/1?id=%23%3CReform%3A%3AFields+email%3D%22soulim%40gmail.com%22%2C+first_name%3Dnil%2C+last_name%3Dnil%3E
```

After patch:

```ruby
form_object = ApplicantForm.new(applicant: applicant, profile: applicant.profile)
url_for(form_object) #=> /applicants/1
```