Correction not to fail with an IllegalArgumentException when using find on an object with an enum as id.

 In IdBinderSimple a call is made to convert the value to the bean type :

    public void bindId(DataBind dataBind, Object value) throws SQLException {
        value = scalarType.toBeanType(value);
        idProperty.bind(dataBind, value);
    }

The cal to toBeanType conflict when using an enum as an id because it'll try to convert something which is already a bean value, and failed with an IllegalArgumentException in EnumToDbValueMap when calling : 

    public Object getBeanValue(T dbValue) {
        if (dbValue == null) {
            return null;
        }
        Object beanValue = valueMap.get(dbValue);
        if (beanValue == null && !allowNulls) {
            String msg = "Bean value for " + dbValue + " not found in " + valueMap;
            throw new IllegalArgumentException(msg);
        }
        return beanValue;
    }

The change is just to check if we already have the correct bean type as done in convertSetId.

The patch was tested on the avaje-ebeanorm-server-3.1.2 tag but not on the master because i'am unable to compile at the time.