CRM-15537 - translate open ended installments to 9999 for authorize.net

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* CRM-15537: editing recurring contribution for authorize.net with unlimited contributions throws error
  https://issues.civicrm.org/jira/browse/CRM-15537
Merge pull request #4463 from jmcclelland/authorize-recur-unlimited

CRM-15537 - translate open ended installments to 9999 for authorize.net