The method document.save() will unseted fields that equal default

[Jennifer-Jia]

The method document.save() will unset fields if that equal ""/[]/{} and equal default. This causes me a lot of trouble. When I get data in mongo through other tools, I can't get the fields and need to set the default value in code. When performing data query, the query conditions will also be affected because the fields will be unset. I hope the method can provide parameters to control whether to unset fields.

[bagerard]

Duplicate of #2462