[References] Allow to reference existing objects with non-numeric PKs

Some entities have non-numeric PK, like currency (USD,BRL..), with this fix now we can do this:

Acme\AcmeBundle\Entity\Payment:
  payment{1..10}:
    currency: BRL
    amount: 10

Thanks.
Merge pull request #40 from vjnrv/patch-1

[References] Allow to reference existing objects with non-numeric PKs