_copyWithSchema(schema) {
    const cl = new SimpleSchema(schema, { ...this._constructorOptions });
    cl._cleanOptions = this._cleanOptions;
    cl.messageBox = this.messageBox.clone();
    return cl;
  }
Copies this schema into a new instance with the same validators, messages,
and options, but with different keys as defined in `schema` argument

@param {Object} schema
@returns The new SimpleSchema instance (chainable)