#3 LdapNode value return result iffy.

[Mark McKenzie]

Here's a tricky one:

You've got an LdapNode, and you want to get a value out
of it (say 'mobile'). The node supports it in the schema (say
'inetOrgPerson').

LdapNode will return the value if its there; otherwise it'll
throw an exception. We should probably check the schema
to see if its an allowed attribute first, and return "" if it is,
otherwise present behaviour.