Clear out tmp/cache when assets:clean is invoked.

Otherwise, if bad data is cached in tmp/clear then the next invocation
of assets:precompile (or a regular incoming request) will pickup files from
tmp without regenerating them from source.

actionpack/lib/sprockets/assets.rake

@@ -16,7 +16,7 @@ namespace :assets do
   end
 
   desc "Remove compiled assets"
-  task :clean => :environment do
+  task :clean => [:environment, 'tmp:cache:clear'] do
     assets = Rails.application.config.assets
     public_asset_path = Rails.public_path + assets.prefix
     rm_rf public_asset_path, :secure => true

railties/test/application/assets_test.rb

@@ -80,7 +80,7 @@ module ApplicationTests
         Dir.chdir(app_path){ `bundle exec rake assets:clean` }
       end
 
-      files = Dir["#{app_path}/public/assets/**/*"]
+      files = Dir["#{app_path}/public/assets/**/*", "#{app_path}/tmp/cache/*"]
       assert_equal 0, files.length, "Expected no assets, but found #{files.join(', ')}"
     end