EF: Fix INSERT INTO with no specified column values, by using DEFAULT VALUES.

Fixes #233.

Instead of running

    INSERT INTO "dbo"."Test"() VALUES () RETURNING "Id"

which results in parse error, run this instead:

    INSERT INTO "dbo"."Test" DEFAULT VALUES RETURNING "Id"

which corresponds to the syntax at http://www.postgresql.org/docs/9.3/static/sql-insert.html.