Disallow setting cookie names that contain '='

[recvfrom]

I think we should add the following to the Set a Cookie section:

1. If |name| contains U+003D (`=`), then return failure.

Otherwise, a cookie with a name containing = will be incorrectly parsed by the server, which I think is more of a bug than a feature. RFC6265bis doesn't explicitly forbid this right now (this case is only possible for cookies not constructed by parsing a cookie line), but I've opened a ticket there for this as well - httpwg/http-extensions#1593