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
You need `::` in front of `Numeric` and `String`. You also don't need to specify `Rational` separately, as it's a subclass of `Numeric`