-Fixed bug where Postgresql JSON type was not able to persist or

-Fixed bug where Postgresql JSON type was not able to persist or
otherwise render a SQL NULL column value, rather than a JSON-encoded
``'null'``.  To support this case, changes are as follows:

* The value :func:`.null` can now be specified, which will always
  result in a NULL value resulting in the statement.

* A new parameter :paramref:`.JSON.none_as_null` is added, which
  when True indicates that the Python ``None`` value should be
  peristed as SQL NULL, rather than JSON-encoded ``'null'``.

Retrival of NULL as None is also repaired for DBAPIs other than
psycopg2, namely pg8000.

fixes #3159