Adding content-length check

It may be that node-memcached checks this, but I'm not seeing where that happens. It appears that it only uses the length which memcached returns to handle the special case of [zero length responses](https://github.com/3rd-Eden/node-memcached/blob/master/lib/memcached.js#L498). As we store the original content-length header we can use it to check the value we get back from memcached.