CRM-15019 - avoid validation error on register date if not using ISO
date format.

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* CRM-15019: Importing participant record with register date in a format other than YYYY-MM-DD results in validation error
  https://issues.civicrm.org/jira/browse/CRM-15019
Merge pull request #3710 from jmcclelland/import-participant-register-date

CRM-15019 - avoid validation error on register date if not using ISO