CRM-19918 Price field "Active on" date ignored in Edit Event Registration > "Change Selections"

Steps to reproduce:

1. Ensure there's an event that's using a price set.
2. Configure one price field in the price set to have a future date in the "Active On" setting.
3. Find a contact who is not registered for this event. Open the contact's record and click "Add Event Registration", then save the new registration. Observe during this process that the price field from step 2 (above) is not displayed in the price options for this registration; this is correct behavior.
4. For this contact, under the Events tab, find the registration for this event, and click the "Edit" link for this event registration. Under "Selections" find and click the "Change Selections" link; this opens the "Change Registration Selections" form. On this form, observe that the price field from step 2 is displayed; this is incorrect behavior, because the price field is configured to be inactive until the given future date.

Extra trouble:
If the price field in question is configured Required=yes, there's no effective way to change selections for any participant in this event, because the "Change Registration Selections" form will require that this price field be selected, even though nobody should be able to select this field until the given future date.