Deprecate ActionView::Base.cache_template_extensions

This commit deprecates unused ActionView::Base.cache_template_extensions option
and adds a deprecation warning for those having it in env specific config files.

Thanks to José Valim for pointing this out.

actionpack/lib/action_view/base.rb

@@ -169,10 +169,10 @@ class Base
     @@cache_template_loading = false
     cattr_accessor :cache_template_loading
     
-    # Specify whether file extension lookup should be cached, and whether template base path lookup should be cached.
-    # Should be +false+ for development environments. Defaults to +true+.
-    @@cache_template_extensions = true
-    cattr_accessor :cache_template_extensions
+    def self.cache_template_extensions=(*args)
+      ActiveSupport::Deprecation.warn("config.action_view.cache_template_extensions option has been deprecated and has no affect. " <<
+                                       "Please remove it from your config files.", caller)
+    end
 
     # Specify whether RJS responses should be wrapped in a try/catch block
     # that alert()s the caught exception (and then re-raises it).