Overwrite stale ZeroOpts= (blank) cookies if HTTPS zero-rated.

* Set-Cookie: ZeroOpts= (blank) necessary for W0-to-W0 user transition.
* When an operator adds HTTPS zero-rating, old objects in cache have stale ZeroOpts= (blank) cookies.
* This patch sidesteps that problem and helps avoid a cache flush while keeping UX consistent.

Change-Id: I7c6cab14e9c082ada3c6b8201a6aae98dd411e61