mime: let FormatMediaType format slash-less media types, to mirror ParseMediaType.

mime: let FormatMediaType format slash-less media types, to mirror ParseMediaType.

A Content-Type always has a slash (type/subtype)
A Content-Disposition does not (e.g. "attachment" or "line").
A "media type" is either one of those, plus optional parameters afterwards.

Our ParseMediaType and FormatMediaType weren't consistent in whether
they permitted Content-Dispositions. Now they both do.

Fixes #11289

Change-Id: Ia75723c9d7adb7f4de0f65482780f823cdadb5bd
Reviewed-on: https://go-review.googlesource.com/17135
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>