net/textproto: reject invalid header keys/values in ReadMIMEHeader

net/textproto: reject invalid header keys/values in ReadMIMEHeader

Return an error when parsing a MIME header containing bytes in the
key or value outside the set allowed by RFC 7230.

For historical compatibility, accept spaces in keys (but do not
canonicalize the key in this case).

For #53188.

Change-Id: I195319362a2fc69c4e506644f78c5026db070379
Reviewed-on: https://go-review.googlesource.com/c/go/+/410714
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>