Handle has one and accepts nil value for it

Now has_one associations will be taken into account when calling to_params in the parent model. 
That allow us to save a parent model containing has_one associations.
For example:

    class Person
      include Her::Model
      has_one :nick_name
      attributes :name
    end

    class NickName
      include Her::Model
      attributes :origin, :author, :created_at
    end

    p = Person.first
    p.assign_attributes(name: 'José', 
      nick_name_attributes: {origin: 'high_school', author: 'Big Guy', created_at: 18.years.ago})
    p.save

Besides that nil value will be accepted:

    p = Person.first
    p.nick_name = nil
    p.save