CRM-13743 use paypal receive date on ipn

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* CRM-13743: receive_date on paypal contribution should be taken from IPN
  http://issues.civicrm.org/jira/browse/CRM-13743

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* CRM-13743: receive_date on paypal contribution should be taken from IPN
  http://issues.civicrm.org/jira/browse/CRM-13743
Merge pull request #1956 from eileenmcnaughton/CRM-13743

CRM-13743 use paypal receive date on ipn