Added foo:install:assets task that copies assets from plugins public directory to application's public directory

This is the most simple and naive approach: just copy every files from engine to app.
The only exception is when file has changed, in that case developer will be
asked if he wants to rename the field.

There is no need to make this task more sophisticated as 3.1 will be shipped
with better assets handling and it will be the default way to handle things.

railties/lib/rails/engine.rb

@@ -546,12 +546,12 @@ def load_seed
             ENV["FROM"] = railtie_name
             Rake::Task["railties:install:migrations"].invoke
           end
-        end
 
-        desc "Copy assets from #{railtie_name} to application"
-        task :assets do
-          ENV["FROM"] = railtie_name
-          Rake::Task["railties:install:assets"].invoke
+          desc "Copy assets from #{railtie_name} to application"
+          task :assets do
+            ENV["FROM"] = railtie_name
+            Rake::Task["railties:install:assets"].invoke
+          end
         end
       end
     end

railties/lib/rails/tasks/railties.rake

@@ -17,4 +17,32 @@ namespace :railties do
       puts "Created symlink #{symlink_path} -> #{path}"
     end
   end
+
+  namespace :install do
+    desc "Copies missing assets from Railties (e.g. plugins, engines). You can specify Railties to use with FROM=railtie1,railtie2"
+    task :assets => :rails_env do
+      require 'rails/generators/base'
+      Rails.application.initialize!
+
+      to_load = ENV["FROM"].blank? ? :all : ENV["FROM"].split(",").map {|n| n.strip }
+      app_public_path = Rails.application.paths["public"].first
+
+      Rails.application.railties.all do |railtie|
+        next unless to_load == :all || to_load.include?(railtie.railtie_name)
+
+        if railtie.respond_to?(:paths) && (path = railtie.paths["public"].first) &&
+           (assets_dir = railtie.config.compiled_asset_path) && File.exist?(path)
+
+          Rails::Generators::Base.source_root(path)
+          copier = Rails::Generators::Base.new
+          Dir[File.join(path, "**/*")].each do |file|
+            relative = file.gsub(/^#{path}\//, '')
+            if File.file?(file)
+              copier.copy_file relative, File.join(app_public_path, assets_dir, relative)
+            end
+          end
+        end
+      end
+    end
+  end
 end

railties/test/railties/shared_tests.rb

@@ -10,6 +10,20 @@ def app
       @app ||= Rails.application
     end
 
+    def test_copying_assets
+      @plugin.write "public/javascripts/foo.js", "doSomething()"
+      @plugin.write "public/stylesheets/foo.css", "h1 { font-size: 10000px }"
+      @plugin.write "public/images/img.png", ""
+
+      Dir.chdir(app_path) do
+        `rake bukkits:install:assets --trace`
+
+        assert File.exists?(app_path("public/bukkits/javascripts/foo.js"))
+        assert File.exists?(app_path("public/bukkits/stylesheets/foo.css"))
+        assert File.exists?(app_path("public/bukkits/images/img.png"))
+      end
+    end
+
     def test_copying_migrations
       @plugin.write "db/migrate/1_create_users.rb", <<-RUBY
         class CreateUsers < ActiveRecord::Migration