net/http/httputil: copy header map if necessary in ReverseProxy

net/http/httputil: copy header map if necessary in ReverseProxy

We were already making a copy of the map before removing
hop-by-hop headers. This commit does the same for proxied
headers mentioned in the "Connection" header.

A test is added to ensure request headers are not modified.

Updates #16875

Change-Id: I85329d212787958d5ad818915eb0538580a4653a
Reviewed-on: https://go-review.googlesource.com/28493
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>