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

- Fix a mistake I made in https://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, as `r->headers_out.cache_control` 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 will
look at that in a follow-up.
downstream-cache-test: deflake the test

As suggested by Anupama, deflake the test by only allowing beaconing
when explitly requested by http request from tests.

Html with beacons in it will bypass the proxy cache.
Merge pull request #671 from pagespeed/oschaaf-downstream-cache-test

downstream-cache-test: deflake the test
Merge pull request #670 from pagespeed/oschaaf-downstream-cache-headers

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