net/textproto: properly trim continued lines in MIME headers

net/textproto: properly trim continued lines in MIME headers

A MIME header can include values defined on several lines.
Only the first line of each value was trimmed.

Make sure all the lines are trimmed before being aggregated.

Fixes #11204

Change-Id: Id92f384044bc6c4ca836e5dba2081fe82c82dc85
Reviewed-on: https://go-review.googlesource.com/15683
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>