Include ETag in HEAD requests

fixes #2083

Conflicts:

	lib/response.js

Note for Purple: This is a cherry-pick of commit
3c7310ebcbee1d65f47e6c6115ade2fa4cbd5b59. The original commit was
done in 4.1.x and we needed it in 3.4.3 which is the version we
use in Purple.