The Transport's Response.Body.Close call was closing the
Response.Body, but the Reader implementation was yielding its buffered
data before returning the error. Add a new method to force an
immediate close.  An audit of the other CloseWithError callers found
that the after-buffered-data behavior was correct for them.

New tests in the main go repo in net/http/clientserver_test.go:
TestResponseBodyReadAfterClose_h1 and TestResponseBodyReadAfterClose_h2

Updates golang/go#13648

Change-Id: If3a13a20c106b5a7bbe668ccb4e3c704a0e0682b
Reviewed-on: https://go-review.googlesource.com/17937Reviewed-by: Russ Cox <rsc@golang.org>