Required model property with default value throws error when property not provided in create

Description / Steps to reproduce / Feature proposal

Given a model property that is defined as required with a default value, trying to create a new model instance and omitting that property from the request yields a 422 error stating the property is missing.

For example, given the following model property definition:

  @property({
    type: 'string',
    required: true,
    default: 'admin',
  })
  username: string;

Trying to create a model instance and omitting the username property yields:

{
    "error": {
        "statusCode": 422,
        "name": "UnprocessableEntityError",
        "message": "The request body is invalid. See error object `details` property for more info.",
        "code": "VALIDATION_FAILED",
        "details": [
            {
                "path": "",
                "code": "required",
                "message": "should have required property 'username'",
                "info": {
                    "missingProperty": "username"
                }
            }
        ]
    }
}

I've recreated the issue using the todo application, and you can view that on my forked feature branch:

https://github.com/aharbis/loopback-next/tree/default-property

I am using Node v10.15.3, and based the recreate on an up-to-date fork of loopback-next.

Current Behavior

The default value defined for the model property is not utilized, and instead a 422 error is thrown stating the required property is missing.

Expected Behavior

If a required property has a default value, and the given property is not provided in the request to create the model instance, the default value should be used.

Finalized agreement:
LoopBack follow the OpenAPI standard and make no interpretations.
required means the user needs to provide the field in the request always, independent of the default.

i.e default: xxx, required: true shouldn't be set at the same time.

Accpetance Criteria

• Update the docs Models.md, add explanations and examples
• Add validation to check if the two fields is set at the runtime