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 should make one in the test.

```ruby
value = Object.new
def value.to_d
  BigDecimal.new("1")
end
```