Rather than specifying every field that should be cleared in Reset,
it is better to just zero the entire struct and only preserve or set the
fields that we actually care about. This ensures that the Header field
is reset for the next use.

Fixes #15077

Change-Id: I41832e506d2d64c62b700aa1986e7de24a577511
Reviewed-on: https://go-review.googlesource.com/21465
Run-TryBot: Joe Tsai <joetsai@digital-static.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>