net/http/httputil: also remove non-standard Proxy-Connection hop-by-hop header

libcurl sends this (despite never being standardized), and the Google
GFE rejects it with a 400 bad request (but only when over http2?).

So nuke it.

Change-Id: I3fc95523d50f33a0e23bb26b9195f70ab0aed0f4
Reviewed-on: https://go-review.googlesource.com/19184


Reviewed-by: Chris Broadfoot <cbro@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>