Fix NULL values with SQLCustomType

An initial fix for SQLCustomType, the current implementation stores 'None' instead of 'NULL'
Example test:
```
    db = DAL('sqlite://memory')
    from gluon.dal.helpers.classes import SQLCustomType
    simple_t = SQLCustomType(
        type = "double",
        native = "double",
    )
    db.define_table('product', 
        Field("price_base", "double"),
        Field("price_custom", simple_t),
    )
    db.product.insert(price_base=None)    
    db.product.insert(price_custom=None)
```
The first insert works, the second fails.