* Fix#4930 workaround for databases that support auto-increment in composite primary key.
* Add test for composite key with auto-increment.
* schema.go: use field.AutoIncrement instead of field.TagSettings["AUTOINCREMENT"], add test to check autoincrement:false
create_test.go: remove unused code: drop table CompositeKeyProduct
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Co-authored-by: Jinzhu <wosmvp@gmail.com>