webdav: set headers *before* calling w.WriteHeader.

http://golang.org/pkg/net/http/#ResponseWriter says that "Changing the
header after a call to WriteHeader (or Write) has no effect."

Change-Id: Ica749e1475a93d535adb1300bc599d5cfe49a94f
Reviewed-on: https://go-review.googlesource.com/3689
Reviewed-by: Nick Cooper <nmvc@google.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>