`ldap`: api.environ["LDAP"] is not a string

This behavior was not expected, but it works from now.
We might consider restricting environment variables setter to only accept strings as values.
If we do so, `ldap` plugin must be upgraded accordingly.

It might be a problem to allow non string types for environment variables because it makes them uneditable with `env` core command.
Anyway, i'll have to think about it.