Support Streaming API

Plack::Middleware::Cached can't cache PSGI with Delayed and Streaming API.

This is my proposition of supporting this API. As I can test, it works with Plack::App::Proxy:

```perl
#!/usr/bin/perl

use Plack::Builder;
use Plack::App::Proxy;
use Plack::Middleware::Cached;

use CHI;

my $cache = CHI->new(
    driver     => 'File',
    serializer => 'YAML',
    root_dir   => 'cache',
    depth      => 0,
);

builder {
    enable 'AccessLog';
    enable 'Cached',
        cache => $cache,
        env   => ['REQUEST_URI'];
    enable 'Proxy::Requests';
    Plack::App::Proxy->new->to_app;
};
```