Not existing optional record element recordPosition leads to exception 

SRU version 1.2 defines the record element recordPosition as optional.

Im my case this element is missing in the response. This leads to the exception:
 
```Fatal error:  Uncaught Scriptotek\Sru\Exceptions\InvalidResponseException: Wrong index of first record in result set. Expected: 1, got: 0```

Is there a way around this? I'm not comfortable to suggest any change in your code.

Thanks!