RFC Errata 4522 (http://www.rfc-editor.org/errata_search.php?eid=4522)
notes that RFC 2616 had a typo in a list of headers that the
httputil.ReverseProxy code copied. Fix the typo in our code.

Fixes #14174

Change-Id: Ifc8f18fd58a6508a02a23e54ff3c473f03e521d3
Reviewed-on: https://go-review.googlesource.com/19133Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>