providers: saml: remove str and unicode conversion
    
    The conversion to str() breaks on unicode strings, since those
    would fall outside of the accepted range of characters.
    Removing this makes Python automatically do the right thing by
    encoding it as str if possible, and unicode otherwise.
    
    Ticket: #213
    Signed-off-by: Patrick Uiterwijk <puiterwijk@redhat.com>
    Reviewed-by: Pierre-Yves Chibon <pingou@pingoured.fr>