Calling response.Body.Close() early would generarate a race before this.
Since closing would return early before the main code path had a chance
to reset the request canceler. Having a non-nil request canceler at the
start of the next request would cause a "request canceled" error.

Here we simply wait for the eofc channel to be closed before returning
from earlyCloseFn, ensuring that the caller won't be re-using that
Request object before we have a chance to reset the request canceler to
nil.

Fixes #21838

Change-Id: I641815526c6ac63d1816c9b6ad49d73715f7a5cb
Reviewed-on: https://go-review.googlesource.com/62891
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>