If there is no sorting query in the HTTP request, and if the sortRecordSet() method is not called, then the specification passed to setDefaultSort() will be used.
This is especially useful if you want the data to already be sorted when a user first see the datagrid.
This method needs to be called before bind().
Sorting specification Structure: array(fieldName => direction, ...)
returns Either true or a PEAR_Error object
throws no exceptions thrown
This function can not be called statically.