Ruby 1.9 compat: ENV.delete rather than assigning nil

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@8452 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

actionpack/test/template/asset_tag_helper_test.rb

@@ -46,7 +46,7 @@ class AssetTagHelperTest < Test::Unit::TestCase
   def teardown
     ActionController::Base.perform_caching = false
     ActionController::Base.asset_host = nil
-    ENV["RAILS_ASSET_ID"] = nil
+    ENV.delete('RAILS_ASSET_ID')
   end
 
   AutoDiscoveryToTag = {