Empty list of references should remove the property entirely
Empty list of references should remove the property entirely

This was causing unexpected results when a reference collection
was going from any entries to empty. The end result in that case
was that the previous state would be maintained (no change).

refs #179

Ocramius/phpcr-odm,valiton-forks/phpcr-odm,AxaliaN/phpcr-odm2,uwej711/phpcr-odm,WouterJ/phpcr-odm,AxaliaN/phpcr-odm2,sarcher/phpcr-odm,WouterJ/phpcr-odm,doctrine/phpcr-odm,Ocramius/phpcr-odm,sarcher/phpcr-odm,doctrine/phpcr-odm,uwej711/phpcr-odm,valiton-forks/phpcr-odm