Sree::Zone, require at least one member

Add validations and specs

app/models/spree/zone.rb

@@ -7,6 +7,8 @@ module Spree
     has_and_belongs_to_many :shipping_methods, join_table: 'spree_shipping_methods_zones'
 
     validates :name, presence: true, uniqueness: true
+    validates :zone_members, presence: true
+
     after_save :remove_defunct_members
     after_save :remove_previous_default
 

spec/models/spree/zone_spec.rb

@@ -3,6 +3,12 @@
 require 'spec_helper'
 
 RSpec.describe Spree::Zone do
+  describe "validations" do
+    it { is_expected.to validate_presence_of(:name) }
+    it { is_expected.to validate_uniqueness_of(:name) }
+    it { is_expected.to validate_presence_of(:zone_members) }
+  end
+
   describe "#match" do
     let(:country_zone) { create(:zone, name: 'CountryZone') }
     let(:country) do