CRM-15380 : Handles #1 and #2.1

CRM-15380 :
1. After saving Change Selections form for a Pending Pay Later event registration, the associated contribution status is updated to Completed even though no payment has been recorded. 
 2. If you select a new price option in Change Selections form (e.g. add 'Planets' checkbox to an existing registration that only had 'Galaxies' in my examples):
2.1 The newly inserted line_item has NULL value for line_item.contribution_id (should have the contribution_id associated with the participant record).