fix stringify problem when multiple headers values exist to be returned

some headers can be present multiple times (ie, Received - or other X-headers).
calling rcube_mime::decode_header/rcube_charset::clean stringifies the return value to "Array" instead of returning the array itself.

for a message with multiple Return-Path headers (which generally shouldn't happen, but it did):

$message->get_header('Return-Path',true); // don't mime-decode header value

would return an array, but

$message->get_header('Return-Path',false); // mime-decode header value

would just return the string "Array"

pull request makes both cases return an array and not a string.