Custom profile fields allow for shortnames containing mixed character
casing, which should also be supported within the user fields API
when retrieving said custom fields.
Given that the DML libs convert all selected columns to lowercased
equivalent, we need to do the same when requesting custom fields to
ensure calling code doesn't try to access `->profile_field_Foo`
property when the DB returns `->profile_field_foo` instead.
9e6bbb4fb8