Could reset expireTime when loading a cached item

``` javascript
apiObj.cache.load("testKey_slow",{ expireTimeMS: expireTime },function(err, load_resp) {
   ...
}
```
The `expireTimestamp` of the cache item will be set to `new Date().getTime() + options.expireTimeMS`.

The `options.expireTimeMS` won't take effect if the initial `expireTimeMS` is null.