Thanks for the suggestion, your PR looks fine functionality-wise!
However, we recently revised the matcher implementations to consistently use a lambda implementation style as well as assertion messages including the cookie name. I've locally added an httpOnly method along your suggestion already, fitting our revised stylistic arrangement.
If you'd like your actual PR to be merged, you'd have to rebase it and align its implementation style. Otherwise I can also proceed with my local changes (not pushed yet, waiting for other stuff to be finished first).