Fix missing version on PATCH passenger/user

[phylor]

It seems that the version is missing when calling update_user in the passenger API.

If the API is called as in the following:

    api_user = Ioki::Model::Passenger::User.new(
      first_name: 'Sarah',
      last_name: 'Musterfrau',
      email: Ioki::Model::Passenger::Email.new(email_address: nil),
      version: 0
    )

    passenger_client.update_user(api_user)

Body sent to the API:

{:data=>
  {:email=>{:email_address=>nil},
   :first_name=>"Sarah",
   :last_name=>"Musterfrau"}}

version is missing in the body and the API complains accordingly:

{"api_errors"=>[{"message"=>"Version missing", "code"=>"version_missing"}],
 "debug_information"=>[]