When setting primary_key belongs_to stop working 

With models like:

```ruby
class Actor
  include Her::Model
  belongs_to :user
end

class User
  include Her::Model
end
```

and api endpoint `/actors/:uid` returning

```json
{
  "uid": "x1",
  "user": { "email": "x1@localhost" }
}
```

this 

```ruby
Actor.find("x1").user.email # => "x1@localhost"
```

works well. But when adding `primary_key :uid` to `Actor` class

```ruby
Actor.find("x1").user.email
# NoMethodError:
#       undefined method `email' for nil:NilClass
```

See the commit for failing spec.