allow ManyToMany associations with referenced column name different from 'id'

[awm1]

Subject

I've found out that if I want to audit entity with ID column with name different from 'id' :

class User
{
/**
* @Orm\Id()
* @Orm\Column(name="user_name",type="string")
/
private $userName;
/*
* @Orm\ManyToMany(targetEntity="AppBundle\Entity\Department", inversedBy="members", cascade={"persist"})
* @Orm\JoinTable(
* name="systemusers_department_membership",
* joinColumns={@Orm\JoinColumn(name="systemusers_id", referencedColumnName="user_name")},
* inverseJoinColumns={@Orm\JoinColumn(name="department_id", referencedColumnName="id")}
* )
*/
private $departmentMembership;
}

...those ManyToMany assoc isn't referred by its correct identifier name, but static 'id' string which will throw Exception. So I try to get it from available source, however I'm not sure if this is the right way to obtain it. I am targeting 1.x branch because this is simple fix which should not break anything.

Changelog

### Fixed
Accessing ManyToMany associations of audited entity with identity field of name different from 'id'.  

[phansys]

Thank you @awm1!

Please, add a test covering this fix.