When http2 fails with ErrNoCachedConn the request is retried with body
that has already been read.

Fixes #25009

Change-Id: I51ed5c8cf469dd8b17c73fff6140ab80162bf267
Reviewed-on: https://go-review.googlesource.com/c/131755
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>