Fetch by id results in exception not nil

The readme describes the following behaviour:
```crystal
# Trying to find a non existent party
Party[2]
# => nil
```

But doing this operation will actually result in a `Ohm::RecordNotFound` exception.
The following is not working:

```crystal
if party = Party[id]
	#work with object
else
	#handle not found
end
```

