participant custom field defaults are not set on additional registrant form

to reproduce:

• create a participant custom data set
• create a field and set a default value
• add the field to a profile to be used for event registrants
• create an event and enable online reg. enable multiple registrants
• use the profile for both the initial and additional registrations
• visit the event registration form and complete the first registrant. select 2 total registrants. advance to the additional registrant form.

note that the custom data field you created will have the default value set correctly on the first registrant's form, but additional registrants will not have this value set.

the bug is present on 4.6.11 (and likely many previous versions) as well as 4.7