CRM-13786 - Fix calc of membership end date when join date is in the past (for fixed period memberships).

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* CRM-13786: Membership End Date not calculated properly for fixed period MembershipTypes
  http://issues.civicrm.org/jira/browse/CRM-13786
Merge pull request #55 from cividesk/CRM-13786

CRM-13786 - Fix calc of membership end date when join date is in the pas...