callback never fired when network is unreachable

[rvagg]

I don't know if this is an ldapjs problem of an ldapauth problem but I have a setup that requires a vpn connection to the ldap server and if the connection isn't running then the server can't be reached and a call to authenticate() never returns on my callback.

I'm having to solve this with a parallel setTimeout() at the moment.

I'm happy to put in some work to sort this out via a pull-request but perhaps you can give some pointers if you know off the top of your head where the problem might be or if this is a known issue?