Issue with nested attributes

When https://github.com/intridea/grape-entity/pull/60 was merged I got several errors on my test suite. This code was running OK in b8c4fe42e411a06898b2cd04b1ad4a064ddea91e. I have created a minimum failing test that can be run on grape-entity to reproduce the issue:

```ruby
require 'spec_helper'

describe "description" do
  it "complex nested attributes" do
    class ClassRoom < Grape::Entity
      expose(:parents, using: 'Parent') { |_| [{}, {}]}
    end

    class Person < Grape::Entity
      expose :user do
        expose(:id) { |_| 'value' }
      end
    end

    class Student < Person
      expose :user do
        expose(:user_id) { |_| 'value' }
        expose(:user_display_id, as: :display_id) { |_| 'value' }
      end
    end

    class Parent < Person
      expose(:children, using: 'Student') { |_| [{}, {}]}
    end

    ClassRoom.represent({}).serializable_hash.should == {
      :parents => [
        { :children =>
          [
            { user: { user_id: 'value', display_id: 'value' } },
            { user: { user_id: 'value', display_id: 'value' } }
          ]
        }
      ]
    }
  end
end
```

This is the error:
````
 1) description complex nested attributes
     Failure/Error: ClassRoom.represent({}).serializable_hash.should == {
     NoMethodError:
       undefined method `[]' for nil:NilClass
     # ./lib/grape_entity/entity.rb:403:in `key_for'
     # ./lib/grape_entity/entity.rb:444:in `block in value_for'
     # ./lib/grape_entity/entity.rb:443:in `each'
     # ./lib/grape_entity/entity.rb:443:in `map'
     # ./lib/grape_entity/entity.rb:443:in `value_for'
     # ./lib/grape_entity/entity.rb:366:in `block in serializable_hash'
     # ./lib/grape_entity/entity.rb:364:in `each'
     # ./lib/grape_entity/entity.rb:364:in `inject'
     # ./lib/grape_entity/entity.rb:364:in `serializable_hash'
     # ./lib/grape_entity/entity.rb:371:in `block (2 levels) in serializable_hash'
     # ./lib/grape_entity/entity.rb:371:in `map'
     # ./lib/grape_entity/entity.rb:371:in `block in serializable_hash'
     # ./lib/grape_entity/entity.rb:364:in `each'
     # ./lib/grape_entity/entity.rb:364:in `inject'
     # ./lib/grape_entity/entity.rb:364:in `serializable_hash'
     # ./spec/grape_entity/error_spec.rb:26:in `block (2 levels) in <top (required)>'
```