downstream-caching: Fix pagespeed resource response headers

- Make kPreserveOnlyCacheControl do what it says on the tin
- Serve pagespeed resources with the response headers computed
  by PSOL regardless of whether downstream caching is configured.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/652
Merge pull request #653 from pagespeed/oschaaf-downstream-caching-headers

downstream-caching: Fix pagespeed resource response headers