- spaces are allowed anywhere but the last character of a boundary

Fixes #18768

Change-Id: I36b054462533ff6dfc060e37e7a58777ae4b66fe
Reviewed-on: https://go-review.googlesource.com/35507
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>