Add Date -> Joi.DateSchema map to ObjectPropertiesSchema<T>

[jamietre]

Handle mapping of DateSchema validation types to Date when using StrictSchemaValidation<T>

For example:

type MyApi = {
   foo: string;
   bar: Date;
}

const schema = Joi.object<MyApi>({
Joi.object({
    foo: Joi.string(),
   // this is a typescript error in current codebase
    bar: Joi.date(),
}