mime: limit UTF-8 encoded-word length to 75 characters

mime: limit UTF-8 encoded-word length to 75 characters

As specified by RFC 2047 section 2, encoded-words may not be more than
75 characters long.

We only enforce this rule when the charset is UTF-8, since multi-bytes
characters must not be split accross encoded-words (see section 5.3).

Fixes #12300

Change-Id: I72a43fc3fe6ddeb3dab54dcdce0837d7ebf658f0
Reviewed-on: https://go-review.googlesource.com/14957
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>