After a response body is closed, we keep writing to the bufPipe. This
accumulates bytes that will never be read, wasting memory. The fix is to
discard the buffer on pipe.BreakWithError.

Updates golang/go#20448

Change-Id: Ia2cf46cb8c401fd8091ef3785eb48fe7b188bb57
Reviewed-on: https://go-review.googlesource.com/43810
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>