api: support the expiry param on all GET requests.

app/logical/application_responder.rb

@@ -3,6 +3,11 @@
 class ApplicationResponder < ActionController::Responder
   # this is called by respond_with for non-html, non-js responses.
   def to_format
+    if get?
+      expiry = request.params["expiry"]
+      controller.expires_in expiry.to_i.days if expiry.present?
+    end
+
     if format == :xml
       options[:root] ||= resource.table_name.dasherize if resource.respond_to?(:table_name)
     end