Clone references before escaping?

As originally discovered in http://www.backup-manager.org/pipermail/dancer-users/2012-April/002424.html :

When `automatic_escaping` is enabled, we recursively encode HTML entities in template params, following references.

Since the app's settings are automatically passed to the template by `Dancer::Template::Abstract`, stored as a reference to the real settings hashref, we can inadvertently go HTML-encoding stuff in the app's actual settings.

A partial fix is for Dancer to clone the settings rather than storing an actual reference, but of course that still means the problem can occur in other cases.

For example:

``` perl
get '/' => sub {
my $foo = { foo => '' };
my $html = template 'bar', { foo => $foo };
# $foo->{foo} has been changed to <Foo>
};
```

Probably a safer fix is for `_encode` to automatically clone any reference it's about to change before making changes. This would need some refactoring in `_encode` to assume that it's starting with a hashref (which it will be) and to pass on both the key and the value each time, rather than simply passing the reference to the value when recursing.

Inizia dal punto di ingresso `_encode` descritto nell’issue e segui il modo in cui elabora ricorsivamente i parametri dei template e i riferimenti. Verifica l’esempio che coinvolge `Dancer::Template::Abstract` e conferma che l’escaping non modifica più le impostazioni originali o i riferimenti `$foo`; aggiungi o esegui i test esistenti pertinenti, se presenti.

Scritto dal modello di indicizzazione a partire dal testo della issue.