net/http/httputil: ReverseProxy forwards Connection headers if first one is empty [1.16 backport]

[gopherbot]

@FiloSottile requested issue #46313 to be considered for backport to the next 1.16 minor release.

@gopherbot please open backport issues for this, according to the security policy.

[dmitshur]

Approving as a fix for a security issue. This backport applies to both 1.16 (this issue) and 1.15 (#46314).

[gopherbot]

Change https://golang.org/cl/323090 mentions this issue: [release-branch.go1.16] net/http/httputil: always remove hop-by-hop headers

[gopherbot]

Closed by merging 0410005 to release-branch.go1.16.