Fix issue with alternate fields not being mapped.

Net::LDAP::Entry returns an empty array if the key does not exist, so
this checks for the key.

This replaces PR #6 by using the code from https://github.com/intridea/omniauth-ldap/commit/b524feb63048591a20de0f70c2d597f0b9d6a156 rather than flattening. The end result is the same.