Bug fixes to 0.9.0

Hey, I just fixed up a couple of the little issues I had when I tried to use your gem out-of-the-box. For what it's worth, it's entirely possible that I only encountered the nil-checking bug because one of my co-workers was messing around with our LDAP server while I was composing and testing my search query, it might not be a problem when working with an operational server.

Implemented a couple of bug fixes:
  * Connection#search now does more graceful nil-checking in its ensure clause, reducing cryptic crash messages
  * Filter class methods should now be able to gracefully accept numeric literals as filter rhs values