is (record) {
    const recordClass = this.recordClass
    return recordClass ? record instanceof recordClass : false
  }
Return whether `record` is an instance of this Mapper's recordClass.

@example
const post = PostMapper.createRecord();

console.log(PostMapper.is(post)); // true
// Equivalent to what's above
console.log(post instanceof PostMapper.recordClass); // true

@method Mapper#is
@param {Object|Record} record The record to check.
@returns {boolean} Whether `record` is an instance of this Mapper's
{@link Mapper#recordClass}.
@since 3.0.0