Fix for issue #1873

Attempt parsing object (ex Moment.js date object) to date. This was implicitly supported <= 0.4.10 but regressed with narrower validation in #1121.

src/core.js

@@ -1042,6 +1042,8 @@ c3_chart_internal_fn.parseDate = function (date) {
         parsedDate = date;
     } else if (typeof date === 'string') {
         parsedDate = $$.dataTimeFormat($$.config.data_xFormat).parse(date);
+    } else if (typeof date === 'object') {
+        parsedDate = new Date(+date);
     } else if (typeof date === 'number' && !isNaN(date)) {
         parsedDate = new Date(+date);
     }