CRM-13880 - Fix testContributionCreateExample to reflect change in contribution create api to set payment_instrument automatically if not provided in params

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* CRM-13880: Make payment instrument required for all contribution, event registration and membership forms
  http://issues.civicrm.org/jira/browse/CRM-13880
Merge pull request #2144 from davecivicrm/CRM-13880b

CRM-13880 - Fix testContributionCreateExample to reflect change in contr...