axes_test: fix SyntaxError

Author: ignamv

test/jasmine/tests/axes_test.js

@@ -2722,7 +2722,7 @@ describe('Test axes', function() {
 
     describe('calcTicks and tickText', function() {
         function mockCalc(ax) {
-            ax = {minexponent: 3, ...ax};
+            ax = Object.assign({minexponent: 3}, ax);
             ax.tickfont = {};
             Axes.setConvert(ax, {separators: '.,', _extraFormat: {
                 year: '%Y',