While ldap_get_entries_moodle() PHPdocs state that it returns "array
ldap-entries with lower-cased attributes as indexes.", this is not true. It
uses ldap_get_attributes() internally, which returns both numerically indexed
attribute names, and dictionary-like entries indexed by attribute names.
Current code lowercases the dictionary-like entries, but then uses the
numerically indexed entries for the attribute names used as keys in the
returned array. The numerically indexed names might or might not be lowercased,
depending on the LDAP server and PHP version) version. E.g., OpenLDAP 2.x,
Novell eDirectory 8.x and MS Active Directory return mixed-cased attribute
names, and PHP 5.x and PHP 7.x don't lowercase them inside ldap_get_entries().
This is probably why all calls to ldap_get_entries_moodle() are followed by
calls to array_change_key_case(), even if that shouldn't be necessary.
So make sure we always return lower-cased attributs as indexes and add some
unit tests to avoid regressions in the future.
While testing the fix, it was found that ldap_stripslashes() was broken and
using a deprecated PCRE patter modifier, so it's also been fixed.
Thanks to Damyon Wiese, some units tests have been added to cover
ldap_addslashes() and ldap_stripslashes().
