RFC 2047 recommends a maximum length of 75 characters for
encoded-words. Due to a bug, encoded-words were limited to 77
characters instead of 75.

Change-Id: I2ff9d013ab922df6fd542464ace70b1c46dc7ae7
Reviewed-on: https://go-review.googlesource.com/20918Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>