CRM-13120 - Reproduce 'null' problem with a core API

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* CRM-13120:
  http://issues.civicrm.org/jira/browse/CRM-13120
CRM-13120 - APIv3 - Don't return "null" from "create" action

 * Split CRM_Core_HTMLInputCoder in three:
   * CRM_Utils_API_AbstractFieldCoder
   * CRM_Utils_API_HTMLInputCoder
   * CRM_Core_HTMLInputCoder (for backward compatibility)
 * Add CRM_Utils_API_NullOutputCoder (based on AbstractFieldCoder)

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* CRM-13120:
  http://issues.civicrm.org/jira/browse/CRM-13120