Adds ModifyResponse, an optional func to ReverseProxy
that modifies a response in the backend, right before
the headers of the response are written to the internal
response writer.
If ModifyResponse returns an error, the proxy returns
a StatusBadGateway error.

Fixes #14237.

Change-Id: I8e03139e34dea0084512ccbd8cc49e941bf9fb5d
Reviewed-on: https://go-review.googlesource.com/32356Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>