split "|"   does throws error

Hi, we found a bug when trying to `split` on a `|` character

```perl
use Template::Liquid;

my $template = Template::Liquid->parse('{% assign fragments = foo | split: "| |" %}{% for fragment in fragments %}frag={{fragment}} {% endfor %}');

print $template->render(foo => "foo| |bar| |baz")
```

Have created a pull request with the fix and multiple tests.