Iitem loader let in empty values different to none

Right now we cant insert values that evaluate `False`.

```python
>>> from scrapy.contrib.loader import ItemLoader
>>> from scrapy.contrib.loader.processor import TakeFirst, Identity
>>> il = ItemLoader(item={})
>>> il.price_in = TakeFirst()
>>> il.add_value('price', 0.0)
>>> il.load_item()
{}
>>> il.price_in = Identity()
>>> il.price_out = TakeFirst()
>>> il.add_value('price', 0.0)
>>> il.load_item()
{'price': 0.0} 
```