Re-encode string value to prevent encoding errors in ruby 1.9+ when saving

```
c:/RailsInstaller/Ruby1.9.3/lib/ruby/gems/1.9.1/gems/activeldap-4.0.0/lib/active
_ldap/attributes.rb:19:in `===': invalid byte sequence in UTF-8 (ArgumentError)
        from c:/RailsInstaller/Ruby1.9.3/lib/ruby/gems/1.9.1/gems/activeldap-4.0
.0/lib/active_ldap/attributes.rb:19:in `blank_value?'
        from c:/RailsInstaller/Ruby1.9.3/lib/ruby/gems/1.9.1/gems/activeldap-4.0
.0/lib/active_ldap/validations.rb:180:in `block in validate_ldap_values'
        from c:/RailsInstaller/Ruby1.9.3/lib/ruby/gems/1.9.1/gems/activeldap-4.0
.0/lib/active_ldap/validations.rb:173:in `each'
        from c:/RailsInstaller/Ruby1.9.3/lib/ruby/gems/1.9.1/gems/activeldap-4.0
.0/lib/active_ldap/validations.rb:173:in `validate_ldap_values'
...
```