generate environment dependent asset digests

If two different environments are configured to use the pipeline, but
one has an extra step (such as compression) then without taking the
environment into account you may end up serving wrong assets

actionpack/lib/sprockets/railtie.rb

@@ -20,6 +20,7 @@ module Sprockets
       app.assets = Sprockets::Environment.new(app.root.to_s) do |env|
         env.static_root = File.join(app.root.join('public'), config.assets.prefix)
         env.logger = ::Rails.logger
+        env.version = ::Rails.env + '-' + env.version
 
         if config.assets.cache_store != false
           env.cache = ActiveSupport::Cache.lookup_store(config.assets.cache_store) || ::Rails.cache

actionpack/test/template/sprockets_helper_test.rb

@@ -205,4 +205,17 @@ class SprocketsHelperTest < ActionView::TestCase
     stubs(:asset_environment).returns(assets)
     assert_match %r{/assets/style-[0-9a-f]+.css}, asset_path("style", "css")
   end
+
+  test "alternate hash based on environment" do
+    assets = Sprockets::Environment.new
+    assets.version = 'development'
+    assets.append_path(FIXTURES.join("sprockets/alternate/stylesheets"))
+    stubs(:asset_environment).returns(assets)
+    dev_path = asset_path("style", "css")
+
+    assets.version = 'production'
+    prod_path = asset_path("style", "css")
+
+    assert_not_equal prod_path, dev_path
+  end
 end