Fix bug that didn't allow showing datepicker twice

When a datepicker was shown, the options date property value was changed to a string, If one tried to show again the datepicker it would throw an error, because we were trying to treat date as a Date object, and it is already a String (objects are mutable in Javascript).

So my solution was to check if the date attribute is a Date object, if so, change to string, if not, leave it as it is.

www/android/DatePicker.js

@@ -26,7 +26,7 @@ DatePicker.prototype.ANDROID_THEMES = {
  */
 DatePicker.prototype.show = function(options, cb, errCb) {
 
-	if (options.date) {
+	if (options.date && options.date instanceof Date) {
 		options.date = (options.date.getMonth() + 1) + "/" +
 					   (options.date.getDate()) + "/" +
 					   (options.date.getFullYear()) + "/" +