This is another attempt at the change attempted in
https://golang.org/cl/27117 and rolled back in https://golang.org/cl/34134

The difference between this and the previous attempt is that this version only
retries if the new field GetBody is set on the Request.

Additionally, this allows retries of requests with idempotent methods even if
they have bodies, as long as GetBody is defined.

This also fixes an existing bug where readLoop could make a redundant call to
setReqCanceler for DELETE/POST/PUT/etc requests with no body with zero bytes
written.

This clarifies the existing TestRetryIdempotentRequestsOnError test (and changes
it into a test with 4 subtests).  When that test was written, it was in fact
testing "retry idempotent requests" logic, but the logic had changed since then,
and it was actually testing "retry requests with no body when no bytes have been
written". (You can confirm this by changing the existing test from a GET to a
DELETE; it passes without the changes in this CL.) We now test for the no-Body
and GetBody cases for both idempotent and nothing-written-non-idempotent
requests.

Fixes #18241
Fixes #17844

Change-Id: I69a48691796f6dc08c31f7aa7887b7dfd67e278a
Reviewed-on: https://go-review.googlesource.com/42142
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>