CIVICRM-429 Billing block fields are not used for duplicate contact checks on Contribution pages

Billing block fields are not used for duplicate contact checks on Contribution pages. This causes duplicate contacts to be created in the database when no Profile has been selected.

Steps to reproduce:

1. Create a Contribution page:
   1. Do not set a Profile for the Contribution page
   2. Enable credit card processing as a payment method
2. Create a contact: David Jones
3. Create a new Individual, Duplicate Contact, Unsupervised Rule: first name, last name
4. Open the Contribution page in test or live mode
   1. Select credit card payment method
   2. Billing block is now displayed, enter the billing information
   3. Enter the contact name: David Jones
   4. Enter other details as required
   5. Submit the contribution
5. Search for contacts in CiviCRM: David Jones
6. In the search results, there are now two duplicate contacts in the database, which is incorrect.
7. The duplicate check should have found and updated the existing contact and only one contact should exist.

The default email field added to the Contribution page is the only field used for duplicate contact checks when there is no Profile set for the Contribution page. All other fields shown in the Billing block are not used.

The solution to this problem is to add a Profile with fields to the Contribution page. The fields on the Profile will then be used to perform the duplicate contact check.

It should be noted that CiviCRM currently will only add a (re)Captcha check to the Contribution page is a Profile is selected, which is highly recommended to reduce credit card exploitation.