DEV: correct implementation of expiry api

Previously we were always hard-coding expiry, this allows the secure session
to correctly handle custom expiry times

Also adds a ttl method for looking up time to live

lib/secure_session.rb

@@ -16,10 +16,14 @@ class SecureSession
 
   def set(key, val, expires: nil)
     expires ||= SecureSession.expiry
-    $redis.setex(prefixed_key(key), SecureSession.expiry.to_i, val.to_s)
+    $redis.setex(prefixed_key(key), expires.to_i, val.to_s)
     true
   end
 
+  def ttl(key)
+    $redis.ttl(prefixed_key(key))
+  end
+
   def [](key)
     $redis.get(prefixed_key(key))
   end