Add a read-only method which plugin authors can use to determine if xss escaping.

This doesn't provide a way to turn off the escaping, but alternative template engine authors
can figure out what their default should be by calling this.  Avoids a messy version + plugin check.

actionpack/lib/action_view/base.rb

@@ -203,6 +203,11 @@ class << self
     @@cache_template_loading = nil
     cattr_accessor :cache_template_loading
 
+    # :nodoc:
+    def self.xss_safe?
+      true
+    end
+
     def self.cache_template_loading?
       ActionController::Base.allow_concurrency || (cache_template_loading.nil? ? !ActiveSupport::Dependencies.load? : cache_template_loading)
     end