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
gzip: enabling gzip on "pagespeed on"

this adds the following configuration on "pagespeed on"
gzip  on;
gzip_proxied any;
gzip_vary on;
gzip_types application/ecmascript;
gzip_types application/javascript;
gzip_types application/json;
gzip_types application/pdf;
gzip_types application/postscript;
gzip_types application/x-javascript;
gzip_types image/svg+xml;
gzip_types text/css;
gzip_types text/csv;
gzip_types text/html;
gzip_types text/javascript;
gzip_types text/plain;
gzip_types text/xml;
gzip_http_version 1.0;

If an explicit configuration is detected the gzip configuration
set by pagespeed on is rollbacked