Hooks not working on mappers

[IAmJulianAcosta]

Description

I'm trying to add some hooks to a mapper this way:

store.defineMapper('event', {
  endpoint: 'events',
  schema: eventSchema,
  relations: eventRelations,
  beforeFindAll: function (id, opts) {
    Mapper.prototype.beforeFindAll.call(this, id, opts, result);
    hooks.push('user beforeFindAll');
  }
});

But the hook is never being called.

I suspect that the problem is: When this [fillIn()](https://github.com/js-data/js-data/blob/v4.0.0-beta.3/src/Mapper.ts#L442) is called, all hooks already have a value, so fillIn won't override the value because it already exists.

Environment

• js-data version: 4.0.0-beta4
• node or browser version: Opera 70
• operating system: OSx