mime/multipart: allow boundary len <= 70

mime/multipart: allow boundary len <= 70

As per RFC 2046, the boundary for multipart MIME is allowed up to 70
characters. The old SetBoundary implementation only allowed up to 69 so
this bumps it to the correct value of 70.

The relevant RFC is at https://www.ietf.org/rfc/rfc2046.txt and section
5.1.1 defines the boundary specification.

Fixes #18793

Change-Id: I91d2ed4549c3d27d6049cb473bac680a750fb520
Reviewed-on: https://go-review.googlesource.com/35830
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>