PROXY: Remove lowercase attribute from save_user()
    
    As this function already receives a struct sss_domain_info * parameter
    as argument, we can simply check whether we will need a lowercase name
    by accessing domain->case_sensitive.
    
    Related:
    https://fedorahosted.org/sssd/ticket/3134
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>