Test for Model.design_document

Change-Id: I1ce2e62c16e7a524e99d1873514ee9e225152324
Reviewed-on: http://review.couchbase.org/26716
Reviewed-by: Andrey Koleshko <ka8725@gmail.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>