Expose setex if exists and is a function

This allows us to use the cache store with a ttl on some transactions,
but not all.

lib/caching.js

@@ -58,6 +58,10 @@ var caching = function (args) {
 
     self.del = self.store.del.bind(self.store);
 
+    if (typeof self.store.setex === 'function') {
+        self.setex = self.store.setex.bind(self.store);
+    }
+
     if (typeof self.store.reset === 'function') {
         self.reset = self.store.reset.bind(self.store);
     }