If during packing, the byte slice gets re-allocated between packing the
ResourceHeader and ResourceBody, the length will get updated in the old
byte slice before re-allocation resulting in a zero length in the final
packed message.

This change fixes the bug by passing the offset at which the length
should be written instead of a slice of the output byte slice from
ResourceHeader encoding step.

Updates golang/go#16218

Change-Id: Ifd7e2f549b7087ed5b52eaa6ae78970fec4ad988
Reviewed-on: https://go-review.googlesource.com/123835
Run-TryBot: Ian Gudger <igudger@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>