DBWrap: Force SQL_MODE=''

Force `SQL_MODE=''` since there are pages that fails if `'STRICT_TRANS_TABLES'` is used.

eg `manage providers.php` fails when:
 * creates a new provider without "Processing time":
 `error 1366: Incorrect integer value: '' for column 'offset_order_close'`
 * goes to list products of a provider:
 `error 1292: Incorrect date value: '0'` at `CALL get products_detail`
Merge pull request #93 from jorix/a-db_wrap

DBWrap: Force SQL_MODE=''