Use URI form of proxy_pass instead of rewrite

[twifkak]

I believe this fixes an issue where a request for e.g. /index%2ehtml would get proxied as /priv/doc/https://server/index.html, and hence fail to adhere to Google's SXG cache requirements.

If it's easy for you to verify this change on your own stack, I would appreciate it. I've made a similar change on my nginx server, but I'm just hand-editing the config file here, so I may have messed something up. If it's not easy, let me know, and I'll try do so on my own machine. Thanks!

[Warashi]

Thank you.

Certainly there is a problem you wrote, and this fixes it.
So I merge this.

By the way, I misread the meaning of amppackager's README?

If the URL points to an AMP page and the AMP-Cache-Transform request header is present, rewrite the URL by prepending /priv/doc and forward the request.

https://github.com/ampproject/amppackager/tree/fadd61811d76ebee041ca0707fc36a7f82de0ab7#productionizing

[twifkak]

Thanks for the feedback! I only discovered this quirk in nginx's rewrite directive 12 days ago. I will think about how to improve the README here. I don't know what's the best method in e.g. Apache or Varnish, but I can point out the general issue, and call out this one known nginx issue.