Add tests for plugin sanity check.

railties/lib/rails/engine/configurable.rb

@@ -3,10 +3,11 @@ class Engine
     module Configurable
       def self.included(base)
         base.extend ClassMethods
-        base.delegate :middleware, :root, :paths, :to => :config
       end
 
       module ClassMethods
+        delegate :middleware, :root, :paths, :to => :config
+
         def config
           @config ||= Engine::Configuration.new(find_root_with_flag("lib"))
         end

railties/lib/rails/plugin.rb

@@ -49,10 +49,9 @@ def config
       eval(File.read(file), binding, file) if file && File.file?(file)
     end
 
-    # TODO Write tests for this sanity check
     initializer :sanity_check_railties_collision do
       if Engine.subclasses.map { |k| k.root.to_s }.include?(root.to_s)
-        raise "The plugin #{name.inspect} is a Railtie or an Engine and cannot be installed as Plugin"
+        raise "\"#{name}\" is a Railtie/Engine and cannot be installed as plugin"
       end
     end
   end

railties/test/plugins/vendored_test.rb

@@ -247,6 +247,30 @@ def index
       assert_equal 200, last_response.status
       assert_equal "Rendered from namespace", last_response.body
     end
+
+    test "plugin cannot declare an engine for it" do
+      @plugin.write "lib/bukkits.rb", <<-RUBY
+        class Bukkits
+          class Engine < Rails::Engine
+          end
+        end
+      RUBY
+
+      @plugin.write "init.rb", <<-RUBY
+        require "bukkits"
+      RUBY
+
+      rescued = false
+
+      begin
+        boot_rails
+      rescue Exception => e
+        rescued = true
+        assert_equal '"bukkits" is a Railtie/Engine and cannot be installed as plugin', e.message
+      end
+
+      assert rescued, "Expected boot rails to fail"
+    end
   end
 
   class VendoredOrderingTest < Test::Unit::TestCase