Ignore AnyEvent::Handler->push_write errors to prevent broken pipe error

Twiggy with Plack::App::Proxy:

```perl
#!/usr/bin/env twiggy

use Plack::Builder;
use Plack::App::Proxy;

builder {
    enable 'AccessLog';
    enable 'Proxy::Connect';
    enable 'Proxy::AddVia';
    enable 'Proxy::Requests';
    Plack::App::Proxy->new->to_app;
};
```

It fails often with following error:

```
127.0.0.1 - - [30/Dec/2013:22:55:16 +0100] "GET http://i.wp.pl/a/i/program_tv/logotypy/tv4.jpg HTTP/1.1" 200 - "http://tv.wp.pl/?ticaid=111ef7" "Mozilla/5.0 (Windows NT 5.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1650.63 Safari/537.36"
AnyEvent::Handle uncaught error: Broken pipe at /home/dexter/perl5/perlbrew/perls/perl-5.18.1/lib/site_perl/5.18.1/Twiggy/Server.pm line 604.
```

It seems that this patch prevents Twiggy crashes.