The default value of "minimize" option should be false

**Do you want to request a *feature* or report a *bug*?**

The Default value of the minimize option should be false.

**What is the current behavior?**
The default value is true.

**If the current behavior is a bug, please provide the steps to reproduce.**

This default behavior will remove empty Object without any notice.

**What is the expected behavior?**
Empty object should remove only when is intented to do so.

**What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.**
mongoose 5.6.6
DB 4.0.10