cookies: do not take cookie name as a parameter

RFC 6265 section 4.2.1 does not set restrictions on cookie names.
This is a follow-up to commit 7f7fcd0d.
Also explicitly check proper syntax of cookie name/value pair.

New test 1155 checks that cookie names are not reserved words.

Reported-By: anshnd at github
Fixes #2564
Closes #2566