Stop mutating date in createDateObjects method

The original method reset the `date` to `startOf('month')` and this can be very confusing. It gave me a headache this afternoon fixing a state issue

Author: florentdestremau

src/createDateObjects.js

@@ -1,7 +1,7 @@
 import moment from 'moment';
 
 export default function createDateObjects(date, weekOffset = 0) {
-  const startOfMonth = date.startOf('month');
+  const startOfMonth = moment(date).startOf('month');
 
   let diff = startOfMonth.weekday() - weekOffset;
   if (diff < 0) diff += 7;