trunk-tracking: update from r3872 to r3895

Squash-merge of @morlovich's work on this.

Note that this does not yet incorprorate the new
pagespeed_admin handler from r3891
Fix improper cache key on write for IPRO of resources with PageSpeed params

We need to be writing with the base name, with params stripped.
This was causing a test to "flake" under valgrind, since with it we got
to the test showing the bug slowly enough for the entry written out by
an earlier test under proper URL to expire.
Merge pull request #651 from pagespeed/morlovich-trunk-tracking-upto-r3895

 trunk tracking upto r3895
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