add deprecation warnings around changes to per-action csp overrides

Author: oreoshake

fixtures/rails_3_2_22_no_init/app/controllers/other_things_controller.rb

@@ -9,6 +9,7 @@ def other_action
   end
 
   def secure_header_options_for(header, options)
+    warn "[DEPRECATION] secure_header_options_for will not be supported in secure_headers 3.x."
     if params[:action] == "other_action"
       if header == :csp
         options.merge(:style_src => "'self'")