Allow specific properties to be `.required`

I have a mySchema with ~10 properties.

In my API, I use mySchema.partial() as all props are optional, BUT one.

I wish I could simply mySchema.partial().required({ myRequired: true }), as partial allows.

It would be a better solution than .partial({ 9 props }) or merging with the mySchema.shape.myRequired.

[ivosabev]

+1 fo this, it is very common scenario.

For example I have a orderSchema and I have an updateOrder() function that takes orderSchema.partial() (listing each key in the partial() function would be too tedious), but I want the orderSchema.shape.id property to be required, so I would like to do orderSchema.partial().required({id:true}) instead of the hack I am currently using orderSchema.partial().omit({id:true}).extend({id: orderIdSchema})

[SrBrahma]

Merged in #1315