Fix host-only cookie duplication & leak

Currently we create duplicate cookies (!) for host-only cookies, one
secure, one insecure. This is because we're always setting the domain
parameter, regardless of the hostOnly value.

See: http://developer.chrome.com/extensions/cookies.html#method-set

Closes issue #159

Signed-off-by: Nick Semenkovich \<semenko@alum.mit.edu\>