dev/membership#10 fix incorrect validation error saying Start date mu…

…st be the same or later than Member since

This problem is because both dates exclude time but the default setting includes it - leading to a
comparison of a datetime with a date+midnight time

CRM/Member/Form/Membership.php

@@ -417,7 +417,7 @@ public function setDefaultValues() {
 
     //setting default join date if there is no join date
     if (empty($defaults['join_date'])) {
-      $defaults['join_date'] = date('Y-m-d H:i:s');
+      $defaults['join_date'] = date('Y-m-d');
     }
 
     if (!empty($defaults['membership_end_date'])) {