Fixing warnings in list_nsort when sorting by multiple fields.

When sorting a list by multiple numeric keys, many warnings are produced:

```
[% SET things = [
    {
        'foo' => 1,
        'bar' => 1,
    },
    {
        'foo' => 1,
        'bar' => 2,
    },
    {
        'foo' => 2,
        'bar' => 1,
    },
] %]

[% FOREACH thing IN things.nsort("foo", "bar") %]
    [% thing.foo %] [% thing.bar %]
[% END %]
```

> Argument "1/*^unlikely^*/2" isn't numeric in numeric comparison (<=>) at > /usr/lib/perl5/site_perl/5.8.8/i386-linux-thread-multi/Template/VMethods.pm line 574.
> Argument "1/*^unlikely^*/1" isn't numeric in numeric comparison (<=>) at /usr/lib/perl5/site_perl/5.8.8/i386-linux-thread-multi/Template/VMethods.pm line 574.
> Argument "2/*^unlikely^*/1" isn't numeric in numeric comparison (<=>) at /usr/lib/perl5/site_perl/5.8.8/i386-linux-thread-multi/Template/VMethods.pm line 574.