title: Validate uniqueness also on deleted documents with Mongoid
body: <p>I've got a rails application with a <code>User</code> model like the following one:</p>
  
  <pre class="lang-rb prettyprint-override"><code>class User
    include Mongoid::Document
    include Mongoid::Timestamps
    include Mongoid::Paranoia
  
    field :email, type: String
    index({ email: 1 }, { unique: true })
  
    validates :email, presence:true, uniqueness:true
  end
  </code></pre>
  
  <p>That's my test:</p>
  
  <pre class="lang-rb prettyprint-override"><code>describe User do
    it 'has an unique email' do
      first_user = Fabricate(:user, email: 'test@fundbase.com')
      first_user.valid?.should be_true # good
      first_user.save
      first_user.delete
  
      second_user = Fabricate(:user, email: 'test@fundbase.com')
      second_user.valid?.should be_true # good
      second_user.save
  
      first_user.restore
      first_user.valid?.should_not be_true # fail
    end
  end
  </code></pre>
  
  <p>Here the last test fails. For some reason the uniqueness of the email field doesn't get checked. Any hints on how to force the validation on restore?</p>