I was implementing rules from RFC 2616. The rules are apparently useless,
ambiguous, and too strict for common software on the Internet. (e.g. curl)

Add more tests, including a test of a chunked request.

Fixes #7625

LGTM=dsymonds
R=golang-codereviews, dsymonds
CC=adg, golang-codereviews, rsc
https://golang.org/cl/84480045