fix mapping for sorting (+ extra sorting param for special sorter mapping) Works for normal and simpleSortMode

override.data.proxy.Server.js

Ext.override(Ext.data.proxy.Server, {
    encodeSorters: function (sorters, string) {
        var model = this.model,
            map = model.prototype.fields.map,
            encSort = Ext.Array.map(sorters, function (sorter) {
                return {
                    property : map[sorter.property].sorting || map[sorter.property].mapping || sorter.property,
                    direction: sorter.direction
                };
            });
        if(string===false){
            return encSort;
        } else {
            return Ext.encode(encSort);
        }

    },
    getParams: function(operation) {
        var me = this;
            params = me.callParent([operation]);

            sorters = operation.sorters,
            simpleSortMode = me.simpleSortMode,
            sortParam = me.sortParam,
            directionParam = me.directionParam;

        if (sortParam && sorters && sorters.length > 0) {
            if (simpleSortMode) {
                var encSort =  me.encodeSorters(sorters, false);
                params[sortParam] = encSort[0].property;
                params[directionParam] = encSort[0].direction;
            }
        }

        return params;
    }
});