Optionally set cache items on POST/PUT

[jpodwys]

When a response comes back from a POST or a PUT, often all of the new/updated asset's information in known by the client. It could be a nice convenience option to be able to have superagent-cache automatically cache the POST/PUT payload, merged with response.body for the given call, if the call is successful. The cache key would simply need to have some fields replaced before adding it to the cache.