Fixes #319. Inserting multiple rows with default values

If an array with one empty object or null etc. is specified knex will now insert the default value correctly for all dialects.

If an array with multiple empty object or null etc.is specified knex will now:
- insert multiple rows with default values for MySQL and derivatives
- insert multiple rows with default values for Oracle and PostgreSQL if a returning column is specified, otherwise **fail**
- **fail** with SQLite.