only call flatJON once before initData()
only call flatJON once before initData()

The data is now flattened before BootstrapTable.initData is
called, and passed in as an argument. The extension was
firing flatJSON multiple times (init, initData and initTable).

Aside from fixing the multiple fires, this fixes:
- The initData extension flattening the data after the sort
  function, causing a bug.
- A bug where getSelections() returns an empty array after
  checkBy().

djhvscf/bootstrap-table-flatJSON,kevinpsiu/bootstrap-table-flatJSON