fix include_missing for nested hash

`include_missing` seems to work only when you have params like that:

````ruby
params do
  requires :first_name, type: String, desc: 'First name'
  requires :last_name, type: String, desc: 'Last name'
end
````

and it does not filter nils If you have nested params like that:

````ruby
params do
  requires :user, :type => Hash do
    requires :first_name, type: String
    requires :last_name, type: String
    requires :address, :type => Hash do
      requires :city, type: String
    end
  end
end
````
This pr is fixing this issue. Once you are ok with the current pr (structure, code), will do the necessary changes to the changelog.