backport header-only fix

Was: trunk-tracking: update to r3632 from 1.7.30.1
test: don't run downstream caching test with native fetcher
native-fetcher: support non-GET request methods like PURGE
native-fetcher: fix to work with nginx 1.5.8+

nginx 1.5.8 changed the resolver api, which the native fetcher uses.

Fixes #578.

Squash-merge of @dinic's #581.
Merge pull request #583 from pagespeed/jefftk-support-purge

native-fetcher: support non-GET request methods like PURGE
Merge branch 'master' into trunk-tracking