Fix rational to decimal on type_cast_from_user

We ran into a problem with Rationals and the `Decimal#cast_value` method. 
For a `Rational` type, use a precision or fallback to default `BigDecimal.new(value, 0)`

@Overbryd and @marianovalles
This more or less kills my plan to remove `limit`, `precision`, and `scale` from the type objects. :\

The `to_i` is unnecessary.