Allow the first query of a prepared statement to use binary encoding

As described [here](http://www.postgresql.org/message-id/CA+=vxNZYNgYQFAr0GJFWOqJp89dr=WejYuOGm10Ni+tG-yY3Dg@mail.gmail.com) - currently you can only do binary transfers from the second query of a prepared statement onwards.