Mongoose 6.x forward compatible

[wirelessr]

Thanks for taking the time to file a pull request with us.
Please take a moment to provide the following details:

When running with Mongoose 5.x and 6.x, there will be an error.

Error: An error occured while parsing schema "test": Unable to parse entity auto: true

Description

The root cause is ObjectId object is a primitive object instead of mongoose.Schema.Types.ObjectId.
Therefore, it has to add a new identifier for ObjectId.

Motivation and context

It can fix #27

How has this been tested?

npm run test

Screenshots (if relevant):

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the CONTRIBUTING document
• I have added tests to cover my changes
• All new and existing tests passed

[wirelessr]

oops, I forgot to merge.