Improper use of net/http.Header.Set

[tgulacsi]

https://github.com/nicolasazrak/caddy-cache/blob/master/cache.go#L25 uses Header.Set to add individual header values, but uses https://godoc.org/pkg/net/http/#Header.Set, which replaces all values with this one - this is wrong.

I'd use

    m := w.Header()
    for k, values := range response.HeaderMap {
        m[k] = append(m[k][:0], v...)
    }

[nicolasazrak]

Thanks it's fixed now in 25084ff I've used headers.Add() which internally do exactly the same.