text: Title: Factory and mutable values - changing the object changes values inside Factory
  Body: Hello, I couldn't find any topic about behavior like this so I create a new one.
  I found this because I started to write some tests for a django project which uses postgresql Jsonb fields.
  
  Every time I change the instance of an object created with Traits, the Factory parameters are change too and all newly created instances have this changed values
  
  Here is a simplified example
  
  ```python
  import factory
  
  
  class A:
      def __init__(self, json_field):
          self.json_field = json_field
  
  
  class AFactory(factory.Factory):
      class Meta:
          model = A
  
      json_field = {'text': 'hello'}
  
      class Params:
          prefilled = factory.Trait(
              json_field={
                  'props': [100, 101, 102, 103, 104],
                  'str_property': 'hello',
              }
          )
  
  
  if __name__ == '__main__':
      item_one = AFactory(prefilled=True)
      print('Item one json_field [created]: {}'.format(item_one.json_field))
      item_one.json_field['props'][0] = 999
      item_one.json_field['str_property'] = 'Goodbye!'
      print('Item one json_field [changed]: {}'.format(item_one.json_field))
      item_two = AFactory(prefilled=True)
      # new instance will have the same json_field as item_one
      print('Item two json_field [created]: {}'.format(item_two.json_field))
      assert item_one.json_field == item_two.json_field
  
      #without traits
      item_three = AFactory()
      print('Item three json_field [created]: {}'.format(item_three.json_field))
      item_three.json_field['text'] = 'different text'
      print('Item three json_field [changed]: {}'.format(item_three.json_field))
      item_four = AFactory()
      print('Item four json_field [created]: {}'.format(item_four.json_field))
      assert item_three.json_field == item_four.json_field
  
  ```
  
  ```bash
  Item one json_field [created]: {'props': [100, 101, 102, 103, 104], 'str_property': 'hello'}
  Item one json_field [changed]: {'props': [999, 101, 102, 103, 104], 'str_property': 'Goodbye!'}
  Item two json_field [created]: {'props': [999, 101, 102, 103, 104], 'str_property': 'Goodbye!'}
  Item three json_field [created]: {'text': 'hello'}
  Item three json_field [changed]: {'text': 'different text'}
  Item four json_field [created]: {'text': 'different text'}
  ```
  
  P.S. at first I wrote about Traits but then I realised that any mutable values assigned inside Factory class are shared between factory and its instances.
  