CRM-13023 - API Case - Allow Get method to be called with no parameters or with entity fields as normal

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* CRM-13023: API Case Entity and Get Action can not be called without search fields
  http://issues.civicrm.org/jira/browse/CRM-13023
Allow API get of Case by ID to use standard get method.
Merge pull request #1974 from rabramley/case-api-standard-get

CRM-13023 - API Case - Allow Get method to be called with no parameters ...