Raise 404 from Admin::UsersController#show if no user found

[Fixes #353]

app/controllers/admin/users_controller.rb

@@ -16,6 +16,7 @@ class Admin::UsersController < Admin::AdminController
 
   def show
     @user = User.where(username_lower: params[:id]).first
+    raise Discourse::NotFound.new unless @user
     render_serialized(@user, AdminDetailedUserSerializer, root: false)
   end
 

spec/controllers/admin/users_controller_spec.rb

@@ -2,7 +2,7 @@ require 'spec_helper'
 
 describe Admin::UsersController do
 
-  it "is a subclass of AdminController" do
+  it 'is a subclass of AdminController' do
     (Admin::UsersController < Admin::AdminController).should be_true
   end
 
@@ -23,13 +23,22 @@ describe Admin::UsersController do
       end
     end
 
-    context '.show' do
+    describe '.show' do
+      context 'an existing user' do
         it 'returns success' do
           xhr :get, :show, id: @user.username
           response.should be_success
         end
       end
 
+      context 'an existing user' do
+        it 'returns success' do
+          xhr :get, :show, id: 'foobar'
+          response.should_not be_success
+        end
+      end
+    end
+
     context '.approve_bulk' do
 
       let(:evil_trout) { Fabricate(:evil_trout) }