code: public function defaults($collection)
      {
          return $this->createFrom(array_replace(Arr::from($collection), $this->items));
      }
docs: Returns a bag with the items from the `$collection` added to the items in this bag
  if they do not already exist by comparing keys. The opposite of {@see replace}.
  
  Example:
  
  Bag::from(['foo' => 'bar'])
  ->defaults(['foo' => 'other', 'hello' => 'world']);
  // => Bag of ['foo' => 'bar', 'hello' => 'world']
  
  @param iterable $collection The collection from which items will be extracted
  
  @return static