lib: allow server.listen({ port: "1234" })

net.connect() accepts `{ port: "1234" }` (i.e. a string) as of commit
9d2b89d0 ("net: allow port 0 in connect()") but net.Server#listen()
did not, creating a minor inconsistency.  This commit rectifies that.

Fixes: https://github.com/iojs/io.js/issues/1111
PR-URL: https://github.com/iojs/io.js/pull/1116


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>