Encode true as xFF

According to the spec: http://tools.ietf.org/html/rfc4511#section-5.1
Force encoding to ASCII-8BIT

We allow this since the library requires Ruby 1.9+
Merge branch 'master' into true-ber
Add BER integration test for true value
Merge pull request #142 from ruby-ldap/true-ber

Encode true as xFF