Handle has-one association

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', craeted_at: 18.years.ago})
    p.save

The json conveyed will have the information to save the nick_name as well as the person's name.

This is actually an implementation of a missing feature documented as a [`TODO`](https://github.com/remiprev/her/blob/3c2a2f0c60f0830041ef5e4cb335160fd5e666af/lib/her/model/parse.rb#L57)