Error with `Double`s without a decimal part

If I define an entity with a `Double` enclosed in an abstract data type (ex. `(Double,Double)`) and I insert a number with the decimal part == 0 (ex. `(1.23, 4.0)`), when I try to get the entity, an error is thrown:

```
*** Exception: PersistMarshalError "field <field name>: Expected Double, received: PersistInt64 <the int number>"
```

I'm using sqlite.
