flush-all does not flush the keys

[szabgab]

use v6;
use Cache::Memcached;

my $memd = Cache::Memcached.new;
$memd.set("foobar", 42);
$memd.flush-all;
say $memd.get("foobar");

prints 42

[jonathanstowe]

This wasm't properly tested and the wrong command was being sent.