Attribute tags should be valid syntax in filters

LDAP [attribute option tags](https://github.com/openldap/openldap/blob/7ca538ff87a4acc022a83c659502d2df49802ad1/doc/rfc/rfc3866.txt#L101-L130) should be valid in filters but currently raise `Net::LDAP::FilterSyntaxInvalidError`

```ruby
Net::LDAP::Filter.construct '(displayName;old=*)'
# => Net::LDAP::FilterSyntaxInvalidError: Invalid filter syntax.
```