[FIX] web_calendar: date/datetime fields format in calendar views

As everywhere else, dates & datetimes must be formatted according
to the format defined in the `res.lang` of the user, and within
the timezone of the user.

Before this revision, when adding a datetime field to a calendar view,
the raw value of the date was displayed, for instance:
`2015-01-01 10:00:00`
instead of
`01/01/2015 11:00:00`
for a user in English(US), in UTC +1.

fixes #5324
opw-653625

addons/web_calendar/static/src/js/web_calendar.js

@@ -510,6 +510,9 @@ openerp.web_calendar = function(instance) {
                             throw new Error("Incomplete data received from dataset for record " + evt.id);
                         }
                     }
+                    else if (_.contains(["date", "datetime"], self.fields[fieldname].type)) {
+                        temp_ret[fieldname] = instance.web.format_value(value, self.fields[fieldname]);
+                    }
                     else {
                         temp_ret[fieldname] = value;
                     }