CRM-20958 - Case, Activity BAOs - Watch out for saving stale timestamps

There appears to be some application logic which follows a process like this:

 1. Read the case
 2. Tweak the data
 3. Save updated case

The problem is comes if step #4 resaves a timestamp loaded in step #1, which
is fairly likely to happen if you read+save the same record.

This was specifically observed on the "Manage Case" screen when editing
activities, but the data-flow is pretty common, so make a general fix to the
BAO.

CRM/Activity/BAO/Activity.php

@@ -285,6 +285,10 @@ public static function deleteActivityContact($activityId, $recordTypeID = NULL)
    * @return CRM_Activity_BAO_Activity|null|object
    */
   public static function create(&$params) {
+    // CRM-20958 - These fields are managed by MySQL triggers. Watch out for clients resaving stale timestamps.
+    unset($params['created_date']);
+    unset($params['modified_date']);
+
     // check required params
     if (!self::dataExists($params)) {
       throw new CRM_Core_Exception('Not enough data to create activity object');

CRM/Case/BAO/Case.php

@@ -89,6 +89,10 @@ public static function add(&$params) {
    * @return CRM_Case_BAO_Case
    */
   public static function &create(&$params) {
+    // CRM-20958 - These fields are managed by MySQL triggers. Watch out for clients resaving stale timestamps.
+    unset($params['created_date']);
+    unset($params['modified_date']);
+
     $transaction = new CRM_Core_Transaction();
 
     if (!empty($params['id'])) {