Drupal views: Activity assignee relationship joins on incorrect column

Steps to reproduce:

1. Create a drupal view of civicrm activities
2. Add a 'Civicrm Activity Assignments' relationship
3. Add civicrm contact display name based on assignee relationship

The 'assignee' will have no correlation to the activity at all.

This is because the the relationship is joining the civicrm_contact and civicrm_activity_assignment tables on their id columns, rather than their id column and assignee_contact_id column, respectively.

The attached patch corrects this.