Fix default value of GitlabSubscription start_date

Makes it Date.today when instantiated and not when the
class was loaded

ee/app/models/gitlab_subscription.rb

 # frozen_string_literal: true
 
 class GitlabSubscription < ApplicationRecord
-  default_value_for :start_date, Date.today
+  default_value_for(:start_date) { Date.today }
 
   belongs_to :namespace
   belongs_to :hosted_plan, class_name: 'Plan'

ee/spec/models/gitlab_subscription_spec.rb

 require 'spec_helper'
 
 describe GitlabSubscription do
-  around do |example|
-    Timecop.freeze do
-      example.run
-    end
-  end
-
   describe 'default values' do
     it do
-      expect(subject.start_date).to eq(Date.today)
+      Timecop.freeze(Date.today + 30) do
+        expect(subject.start_date).to eq(Date.today)
+      end
     end
   end