Make Model.expire act like the other options, and add tests for it

lib/redistat/model.rb

@@ -46,8 +46,12 @@ module Redistat
 
     alias :class_name :scope
 
-    def expire(exp)
-      options[:expire] = exp.is_a?(Hash) ? exp : Hash.new(exp)
+    def expire(exp = nil)
+      if !exp.nil?
+        options[:expire] = exp.is_a?(Hash) ? exp : Hash.new(exp)
+      else
+        options[:expire]
+      end
     end
 
     def connect_to(opts = {})