dgram: handle default address case when offset and length are specified

Fixes a regression introduced by: https://github.com/nodejs/node/pull/4374.
Adds a new test to avoid similar issue in the future.
The test is disabled on windows, because this feature never worked
there.

Fixes: https://github.com/nodejs/node/issues/5398


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Evan Lucas <evanlucas@me.com>