Using numerics any reference name causes error with v1.8.1

[mssimi]

Bug Report

Q	A
Version	1.8.1

Summary

After upgrade from 1.7.0 to 1.8.1 reference name that is numeric does not work, even when it is typed string.

Current behavior

Using numeric reference name causes error with 1.8.1, but not with 1.7.0

Expected behavior

Using numeric reference name working for both 1.7.0 and 1.8.1

How to reproduce

With 1.8.1

$this->setReference('a12345', $anyEntity); // works
$this->setReference('12345', $anyEntity); // throws error 

 Doctrine\Common\DataFixtures\ReferenceRepository::setReferenceIdentity(): A  
  rgument #1 ($name) must be of type string, int given, called in /srv/vendor  
  /doctrine/data-fixtures/src/Event/Listener/ORMReferenceListener.php on line  
   52                       

With 1.7.0

$this->setReference('a12345', $anyEntity); // works
$this->setReference('12345', $anyEntity); // works too