I both forgot that this list could contain duplicates, and I had
forgot to run the net/http tests against CL 123615 before mailing it,
which ended up catching this bug.

The diff of this file from the commit before CL 123615 (a45b4abe^ ==
cffdcf67) to this commit is:

    https://gist.github.com/bradfitz/0b7abf8fa421515aed9c4d55ce3a1994

... effectively reverting much of CL 123615 and just moving the 1xx
handling down lower.

Updates golang/go#26189 (fixes after vendor into std)
Updates golang/go#17739

Change-Id: Ib63060b0bb9721883b4b91a983b6e117994faeb9
Reviewed-on: https://go-review.googlesource.com/123675
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>