fix include_missing for nested hash

[georgimitev]

include_missing seems to work only when you have params like that:

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:

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.

[dblock]

This makes sense. We actually don't seem to document include_missing at all. Would you mind also update this in the README?

[georgimitev]

@dblock i've updated the README and CHANGELOG, I've also added a section about declared(params) since i couldn't find one. Let me know if anything else is required.

[dblock]

This is great, merging.