mime: add .json to built-in list of MIME types

Since json is popular and mime package's builtin type does not contain
it, and some Linux distributions do not contain the '/etc/mime.types' file
with minimal installations.

Change-Id: I933393c82be296ef176206c253f4dd19b6f33bb1
GitHub-Last-Rev: ce4eae56a4d468631555dc85e0e7ab4bbd0ef45d
GitHub-Pull-Request: golang/go#34737
Reviewed-on: https://go-review.googlesource.com/c/go/+/199657


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>