The struct XMLName sets the default namespace, but
that's not good enough for nested tags, because an
earlier tag can set the implicit parents of a subsequent
tag. This change makes sure that we always explicitly set the
namespace on a tag when possible.

See https://go-review.googlesource.com/#/c/5910/4/src/encoding/xml/marshal_test.go@628
for discussion.

Change-Id: If1afc536471c0be83e5dd80381b598476ea3f44d
Reviewed-on: https://go-review.googlesource.com/6927Reviewed-by: Nigel Tao <nigeltao@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>