Add option to use different backend in Plack::App::Proxy

Hi Ingy,

I ran into an issue where AnyEvent::HTTP mangled multiple Set-Cookie headers. The LWP backend of Plack::App::Proxy behaves better in that respect. Since I prefer using your ProxyMap over raw Proxy, I needed to be able to specify the different backend. This patch accomplishes that.

Thanks for your time,

Rhesa