Add a deprecation warning about config.action_view.debug_rjs.

Upgrading to 3.1 removes debug_rjs and upgraded apps will fail to start up.

actionpack/lib/action_view/base.rb

@@ -165,8 +165,12 @@ module Subclasses
 
     # Specify whether RJS responses should be wrapped in a try/catch block
     # that alert()s the caught exception (and then re-raises it).
-    cattr_accessor :debug_rjs
+    cattr_reader :debug_rjs
     @@debug_rjs = false
+    def self.debug_rjs=(new_value)
+      ActiveSupport::Deprecation.warn("config.action_view.debug_rjs will be removed in 3.1, from 3.1 onwards you will need to install prototype-rails to continue to use RJS templates ")
+      @@debug_rjs = new_value
+    end
 
     # Specify the proc used to decorate input tags that refer to attributes with errors.
     cattr_accessor :field_error_proc

actionpack/test/template/prototype_helper_test.rb

@@ -435,11 +435,15 @@ def test_collection_proxy_with_each_slice
   end
 
   def test_debug_rjs
-    ActionView::Base.debug_rjs = true
+    assert_deprecated /prototype-rails/ do
+      ActionView::Base.debug_rjs = true
+    end
     @generator['welcome'].replace_html 'Welcome'
     assert_equal "try {\n$(\"welcome\").update(\"Welcome\");\n} catch (e) { alert('RJS error:\\n\\n' + e.toString()); alert('$(\\\"welcome\\\").update(\\\"Welcome\\\");'); throw e }", @generator.to_s
   ensure
-    ActionView::Base.debug_rjs = false
+    assert_deprecated /prototype-rails/ do
+      ActionView::Base.debug_rjs = false
+    end
   end
 
   def test_literal