CRM-6602 If event registration form doesn't inlude name fields in a profile or billing block and existing contact is matched then the contact display name is overwritten by email

In this case any value in contact.display_name is replaced with the email address. (We don't mess up the existing first, middle, last name data - so the display name "cache" can be restored by re-saving the record.)

Steps:

• Created new event using sample template "Free Event with Online Registration"
• Modified that event so no profile was used (only email address collected during registration)
• Identify and existing contact who has first and last name, and grab their email address
• log out
• Register for the event using the copied existing contact email
• log back in and look at Contact Summary for that contact - display name will be modified to be email address