Define the header value parser

[annevk]

Although #6 is closed, #6 (comment) still seems relevant:

Another consideration that comes up routinely when dealing with HTTP fields is the potential for them to be repeated, which can happen by accident more often than expected. Under those conditions, it is possible that the values are combined.

In this case, that could produce results like Sec-GPC: 1, 1 at a receiving entity. A simple parser that does simple comparisons (headers.get("sec-gpc") == "1") will make a mistake.

(Although TBD if that's a mistake or not I suppose.)