Content-Base – absolute URL

[bAndie91]

https://github.com/swiperproxy/swiperproxy/blob/c84badcb58a46c886e744eee2fabff7b5da62d9c/Proxy.py#L332

1. Content-Base must be absolute url, see rfc2068.
2. append content-base header provided by the backend (resp.getheader('content-base')) to ours.