net/http: make SameSiteDefaultMode behavior match the specification

The current specification does not foresee a SameSite attribute without
a value. While the existing implementation would serialize SameSite in a
way that would likely be ignored by well-impelemented clients, it is
better to not rely on this kind of quirks.

Specification: https://tools.ietf.org/html/draft-ietf-httpbis-rfc6265bis-05#section-4.1.1

Fixes #36990

Change-Id: Ie51152741d7e84bab64d3e4e4f780286932acbde
Reviewed-on: https://go-review.googlesource.com/c/go/+/256498


Trust: Roberto Clapis <roberto@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>