As Andy Balholm noted in #11207:

"RFC2616 §4.2 says that a header's field-content can consist of *TEXT,
and RFC2616 §2.2 says that TEXT is <any OCTET except CTLs, but
including LWS>, so that would mean that bytes greater than 128 are
allowed."

This is a partial rollback of the strictness from
https://golang.org/cl/11207 (added in the Go 1.6 dev cycle, only
released in Go 1.6beta1)

Fixes #11207

Change-Id: I3a752a7941de100e4803ff16a5d626d5cfec4f03
Reviewed-on: https://go-review.googlesource.com/18374Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>