Fix propagationDelay being dropped for challenges

plugins.js

@@ -183,6 +183,9 @@ P._normalizeChallenge = function(name, ch) {
         }
         gch.get = wrappy(ch.get);
     }
+    if (ch.propagationDelay) {
+        gch.propagationDelay = ch.propagationDelay;
+    }
 
     return gch;
 };