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
Alternative implementation that would actually work:

```ruby
def cast_value(value)
  case value
  when Numeric, String
    BigDecimal(value, precision.to_i)
  else
    if value.respond_to?(:to_d)
      value.to_d
    else
      cast_value(value.to_s)
    end
  end
end
```