Preserve order of keys requested via getMulti()

The php-memcached extension has nice feature to 'preserve_order' when they initialize the result object. It uses nulls in the same order as the keys, and later replaces them, because multiget responses could go in arbitrary order:
https://github.com/php-memcached-dev/php-memcached/blob/master/php_memcached.c#L744-L746

(thanks, avsej for finding this!)