Don't print "<nil>" when IP cannot be parsed

We only enter this condition when `receivedHeaderIP` is `nil` which results
in the error message saying `with non-IP value "<nil>"`.

Instead print the original unparsed value. So that it's clearer why a
non-empty header value couldn't be validated as an IP address.
Merge pull request #18 from alphagov/fix_client_ip_error

Don't print "<nil>" when IP cannot be parsed