Add test of admin editing a user

Fails with:

    1) User admin behaves like admin ability edits a user
       Failure/Error: raise ActionController::MissingExactTemplate, message

       ActionController::MissingExactTemplate:
         Admin::UsersController#edit is missing a template for request formats: text/html

re #2975

spec/features/user_spec.rb

@@ -3,12 +3,24 @@
 require 'spec_helper'
 
 feature User do
+  let(:admin) { create(:admin) }
+  let!(:user) { create(:user) }
 
   shared_examples 'admin ability' do
-
+    scenario 'edits a user', feature: true, js: true do
+      visit admin_users_path
+      within "tr#user_#{user.id}" do
+        click_on 'Edit'
+      end
+      fill_in 'Name', with: 'Edited Name'
+      click_button 'Update User'
+      expect(flash).to include('Updated Edited Name')
+    end
   end
 
   describe 'admin' do
+    before { sign_in admin }
+
     it_behaves_like 'admin ability', :admin
   end
 end