downstream-caching-headers: make nginx 1.5.9+ pass tests as well

- Fix a mistake I made in ttps://github.com/pagespeed/ngx_pagespeed/pull/653
  This is the primary reason the test starts failing, the base fetch
  handled `kPreserveOnlyCacheControl` in the wrong way.
- Use `ps_set_cache_control` when copying a Cache-Control header
  to nginx. This is where modules running after us might expect it
- Exclude a few more lines from curl's verbose output in the keep-
  alive tests that I came accross when testing on ubuntu 13.10
  Desktop.

Note that this does not deflake the downstream cache test yet, I'll
look at that in a follow-up.