3.1 and 3.2 disagree with 4.1

[mikewest]

@jeremy's comment in rack/rack#1051:

Draft 7 is a little confusing now. It allows SameSite with no attribute value in 3.1 and says that cookies with an invalid SameSite value should be treated as Strict in 3.2, yet 4.1 makes it clear that User Agents must ignore cookies with a SameSite attribute whose value is not Strict or Lax.

3.1

samesite-av = "SameSite" / "SameSite=" samesite-value

3.2

If the "SameSite" attribute's value is "Strict", or if the value is invalid, the cookie will only be sent along with "same-site" requests.

4.1

If "cookie-av"'s "attribute-value" is not a case-sensitive match for "Strict" or "Lax", ignore the "cookie-av".

The text in 4.1 is correct, and I should audit the rest of the doc to make sure the change is clear.