Purposes Extension: Exclude comma from the DNT-Consent syntax

[royfielding]

Since HTTP uses the comma for joining header fields that are received multiple times, it can be ambiguous when the field syntax is allowed to contain bare (unquoted) commas. That's why most HTTP header fields use semicolons where one would normally use commas in English.

The experts usually recommend that comma be excluded from the field syntax of new header fields. IOW, exclude %2C:

        DNT-Consent = %x21-7E ; excludes CTL, SP

changes to

        DNT-Consent = %x21-2B / %x2D-7E ; excludes CTL, SP, ","

[royfielding]

and change the example in 3.2.1 that uses a comma