Fix Staff email invite

There is an issue when inviting a staff user where the check for the
AnonymousSupportUser class doesn't work as expected.

Rather than check on class, we can use duck-typing and check for the
presence of an expected method.

This means the code is less coupled to a specific class and is more
explicit about the expectations for the code branch.

app/controllers/staff/invitations_controller.rb

@@ -7,7 +7,7 @@ class Staff::InvitationsController < Devise::InvitationsController
   protected
 
   def invite_resource(&block)
-    if current_inviter.is_a?(AnonymousSupportUser)
+    if !current_inviter.respond_to?(:primary_key)
       resource_class.invite!(invite_params, &block)
     else
       super

spec/system/support/staff_invite_as_staff_member_spec.rb

@@ -0,0 +1,75 @@
+require 'rails_helper'
+
+RSpec.feature 'Staff support', type: :system do
+  include CommonSteps
+
+  scenario "Staff user invites another Staff member" do
+    given_the_service_is_open
+    and_a_staff_member_exists
+    and_i_am_logged_in_as_a_staff_member
+    when_i_visit_the_staff_page
+    then_i_see_the_staff_index
+
+    when_i_click_on_invite
+    then_i_see_the_staff_invitation_form
+
+    when_i_fill_the_invitation_form
+    and_i_send_invitation
+    then_i_see_an_invitation_email
+    then_i_see_the_staff_index
+    then_i_see_the_invited_staff_user
+  end
+
+  private
+
+  def and_a_staff_member_exists
+    create(:staff, :confirmed)
+  end
+
+  def and_i_am_logged_in_as_a_staff_member
+    visit new_staff_session_path
+    fill_in 'Email', with: '[email protected]'
+    fill_in 'Password', with: 'password'
+    click_button 'Log in'
+  end
+
+  def when_i_visit_the_staff_page
+    visit support_interface_staff_index_path
+  end
+
+  def then_i_see_the_staff_index
+    expect(page).to have_current_path('/support/staff')
+    expect(page).to have_content('Staff')
+  end
+
+  def when_i_click_on_invite
+    click_link 'Invite staff user'
+  end
+
+  def then_i_see_the_staff_invitation_form 
+    expect(page).to have_current_path('/staff/invitation/new')
+    expect(page).to have_content('Send invitation')
+  end
+
+  def when_i_fill_the_invitation_form
+    fill_in 'Email', with: '[email protected]'
+  end
+
+  def and_i_send_invitation
+    click_button 'Send an invitation'
+  end
+
+  def then_i_see_an_invitation_email
+    perform_enqueued_jobs
+    message = ActionMailer::Base.deliveries.first
+    expect(message).to_not be_nil
+
+    expect(message.subject).to eq("Invitation instructions")
+    expect(message.to).to include("[email protected]")
+  end
+
+  def then_i_see_the_invited_staff_user
+    expect(page).to have_content("[email protected]")
+  end
+end
+