Encode true as xFF

According to the spec (http://tools.ietf.org/html/rfc4511#section-5.1), we're currently encoding `true` incorrectly as `\x01\x01\x01` instead of `\x01\x01\xFF`.

Fixes #31.

Wouldn't mind finding a good integration test for this, but nothing obvious or direct comes to mind.

cc @jch @schaary