[Bug] BITMAP internal _type error when the bitmap is being union or add

[HangyuanLiu]

bitmap internal _type error when the bitmap is being union or add.
When multiple bitmap are union, the type of the bitmap should not be modified if the result is still single.

[morningman]

hi @HangyuanLiu , next time, you can link the issue to the PR by add Fix #3748 in your PR's comment. So that the issue will be automatically closed after PR being merged.

[HangyuanLiu]

hi @HangyuanLiu , next time, you can link the issue to the PR by add Fix #3748 in your PR's comment. So that the issue will be automatically closed after PR being merged.

OK，get it