encoding/json: explicitly document and test "-" key tag

Struct fields can be suppressed in JSON serialization by "-" tags, but
that doesn't preclude generation of "-" object keys.
Document and verify the mechanism for doing so.

Change-Id: I7f60e1759cfee15cb7b2447cd35fab91c5b004e6
Reviewed-on: https://go-review.googlesource.com/21204


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>