CRM-13121 - CRM.api - Always encode data with JSON

The query-string encoder misencodes empty arrays -- and most likely
misencodes other things. I'm not sure why one would ever prefer the
query-string encoder over the JSON encoder.

Ping @TechToThePeople @colemanw

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* CRM-13121: CRM.api cannot call "replace" with empty list
  http://issues.civicrm.org/jira/browse/CRM-13121