Change Selections postProcess: erroneously updates contribution status to Completed and inserts line_items with NULL contribution_id

Two related bugs occur when using the Change Selections form:
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.

Screenshot:
ChangeSelectionPostProcessIncorrectlyUpdatesContributionStatus.PNG

NOTE: This is somewhat related to issue # 3 in CRM-15364 - in that case using Participant form in Edit mode and checking Record Payment correctly updates contribution status to completed (but doesn't create the expected linked financial transactions - so the Total Paid when viewing the participant record is still $0).

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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).
2.2 If the original transaction has a NOT NULL fee_amount and net_amount, the value of the net_amount needs to be updated. Currently, after the transaction the net_amount still contains the original value of ($previous-total - $fee_amount).

Screenshots: ViewContributionAfterChangeSelections_MissingLineItemANDNetAmountWrong.PNG
ChangeSelections-AddItem_LineItem_ContributionID_Missing.PNG