Use `#bytesize` instead of `#size` when checking for cookie overflow

Although the cookie values happens to be ASCII strings because they are
Base64 encoded, it is semantically incorrect to check for the number of the
characters in the cookie, when we actually want to check for the number of the
bytes it consists of.

Furthermore it is unecessary coupling with the current implementation that
uses Base64 for encoding the values.
Merge pull request #16133 from Agis-/cookie_overflow_check

Use `#bytesize` instead of `#size` when checking for cookie overflow