mockCalc: ensure ax is modified in-place

Author: imartinezvazquez

test/jasmine/tests/axes_test.js

@@ -2722,7 +2722,8 @@ describe('Test axes', function() {
 
     describe('calcTicks and tickText', function() {
         function mockCalc(ax) {
-            ax = Object.assign({minexponent: 3}, ax);
+            if (ax.minexponent === undefined)
+                ax.minexponent = 3;
             ax.tickfont = {};
             Axes.setConvert(ax, {separators: '.,', _extraFormat: {
                 year: '%Y',