Revert "Revert "[Tests] DayPicker: unit test for getWeekHeaders()""

This reverts commit defb7fd.

Author: majapw

test/components/DayPicker_spec.jsx

@@ -3,6 +3,7 @@ import moment from 'moment/min/moment-with-locales';
 import { expect } from 'chai';
 import sinon from 'sinon-sandbox';
 import { mount, shallow } from 'enzyme';
+import { cloneDeep } from 'lodash';
 
 import * as isDayVisible from '../../src/utils/isDayVisible';
 import isSameMonth from '../../src/utils/isSameMonth';
@@ -1013,6 +1014,23 @@ describe('DayPicker', () => {
     });
   });
 
+  describe('#getWeekHeaders', () => {
+    it('returns unmutated weekday headers for currentMonth in a future', () => {
+      sinon.stub(PureDayPicker.prototype, 'render').returns(null);
+
+      const getWeekHeadersSpy = sinon.spy(PureDayPicker.prototype, 'getWeekHeaders');
+      const INITIAL_MONTH = moment().add(2, 'Months').week(3).weekday(3);
+      const wrapper = shallow(<DayPicker initialVisibleMonth={() => INITIAL_MONTH} />).dive();
+      const instance = wrapper.instance();
+      const state = cloneDeep(wrapper.state());
+
+      expect(instance.getWeekHeaders()).to.be.eql(INITIAL_MONTH.localeData().weekdaysMin());
+      expect(instance.state).not.to.equal(state);
+      expect(instance.state).to.eql(state);
+      expect(getWeekHeadersSpy).to.have.property('callCount', 1);
+    });
+  });
+
   describe('life cycle methods', () => {
     describe.skip('#componentDidMount', () => {
       describe('props.orientation === HORIZONTAL_ORIENTATION', () => {